Merge bitcoin/bitcoin#25369: Unsubtree Univalue

d873ff96e51a3e7f2fdc3fdd1baee2bbe7583e06 refactor: cleanups post unsubtree'ing univalue (fanquake)
e2aa7047f9b94232ff4958381321e6867d24b379 refactor: un-subtree univalue (fanquake)

Pull request description:

  At this point, maintaining Univalue as a subtree doesn’t serve much purpose, other than being an inconvenience for making changes to the code (along with polluting our repo with a number of files we don’t use). Our [Univalue fork](https://github.com/bitcoin-core/univalue-subtree) currently deviates from the [upstream API](https://github.com/jgarzik/univalue), and for some time has been marked as not-maintained for use by other projects (I'm not aware of any that use it). The upstream Univalue is not maintained, and has not been for some time. There are no new releases, bugs remain unfixed, and PR's we've upstreamed, https://github.com/jgarzik/univalue/pulls, are not being commented on/merged.

  Another substantial benefit of no-longer maintaining a subtree is removing the rather awkward work-flow currently required to make changes to the Univalue code, particularly breaking changes / introducing new features, e.g. https://github.com/bitcoin-core/univalue-subtree/pull/27. We need to dance around and merge changes to our fork, with a flag, then pull them down here, then switch to using the new code, then go back to our Univalue repo, and remove the old code / flag, then pull the repo down here again, and remove our usage of the flag. Quite the overcomplicated mess.

  With this PR I'm proposing we stop treating Univalue like a subtree, or upstream project/fork, and going forward, treat it as part of this codebase, which we can refactor directly (with pulls to this repo. Ideally, after this is merged, our univalue subtree repo could be marked as "archived". In this repo, I think there is a good chance that the Univalue code will ultimately be refactored away into "modern" C++, i.e using `std::variant` (at least one person has played around with doing this).

  Univalue history:
  - Subtree first introduced: https://github.com/bitcoin/bitcoin/pull/6637
  - `--system-univalue` option introduced: https://github.com/bitcoin/bitcoin/pull/7349
    Suggestion was to use system Univalue by default.
    This was pushed back on by contributors, as well as the [upstream Univalue](https://github.com/jgarzik/univalue) maintainer (jgarzik).
  - Our fork's README was updated to say `It is not maintained for usage by other projects. Notably, the API may break in non-backward-compatible ways.` : https://github.com/bitcoin-core/univalue-subtree/pull/17
  - Our fork README additionally updated to say `the API is broken in non-backward-compatible ways.` : https://github.com/bitcoin-core/univalue-subtree/pull/30
  - `--system-univalue` option removed: https://github.com/bitcoin/bitcoin/pull/22646
  - Univalue "subtree" removed: This PR.

  Guix Build (x86_64):
  ```bash
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  e450bd82020d5086f3bb0a23181263315cc05eaf6e5809d0a2115bff4e7ddb2e  guix-build-d873ff96e51a/output/powerpc64le-linux-gnu/bitcoin-d873ff96e51a-powerpc64le-linux-gnu.tar.gz
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  cd219fab8918b061a342357d298aca0c044feb34c6d50a7851d5d3bf18cec267  guix-build-d873ff96e51a/output/x86_64-apple-darwin/bitcoin-d873ff96e51a-x86_64-apple-darwin-unsigned.dmg
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  5703b02c2647f9997aa5ca12514d6a54b1eb2e29046223ca062383326b95894f  guix-build-d873ff96e51a/output/x86_64-linux-gnu/bitcoin-d873ff96e51a-x86_64-linux-gnu-debug.tar.gz
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  5d180b0415fa8e825d46928c168cb1ae6e27016841b2ff8e190bf13879a5545c  guix-build-d873ff96e51a/output/x86_64-w64-mingw32/SHA256SUMS.part
  d469695a32f6414b25fef7b5fdfda4d854071450ba25148a1dce468114fa9057  guix-build-d873ff96e51a/output/x86_64-w64-mingw32/bitcoin-d873ff96e51a-win64-debug.zip
  2e7d4e533a5998863c115c586c61b75b4039cd329e12ed24cff78b7f16b6ea57  guix-build-d873ff96e51a/output/x86_64-w64-mingw32/bitcoin-d873ff96e51a-win64-setup-unsigned.exe
  3dabbd627b532beef57c3d4b5bd30c93c5ea74c492918484cf24685aca8d7bc4  guix-build-d873ff96e51a/output/x86_64-w64-mingw32/bitcoin-d873ff96e51a-win64-unsigned.tar.gz
  3a40660fba08f7632efd1f73c198f8298db33eab6ef5eaca88b997d95fc31f29  guix-build-d873ff96e51a/output/x86_64-w64-mingw32/bitcoin-d873ff96e51a-win64.zip
  ```

  Guix Build (arm64):
  ```bash
  0e764679199358fc321dcfcb58c6302e6518f55b3fd27bdd47f2da2a826ba16a  guix-build-d873ff96e51a/output/arm-linux-gnueabihf/SHA256SUMS.part
  5955d28e6d56e5a3297dab723b8478f1b0bb7f5b86476c581339122f34cc7f14  guix-build-d873ff96e51a/output/arm-linux-gnueabihf/bitcoin-d873ff96e51a-arm-linux-gnueabihf-debug.tar.gz
  49c68bc0066f709be68f1e5731425d51fb3cb8062a24aa9fa599987165759cad  guix-build-d873ff96e51a/output/arm-linux-gnueabihf/bitcoin-d873ff96e51a-arm-linux-gnueabihf.tar.gz
  ca678d4eb27c9fa3c527211c0ccb145322a15f327545b5c82f1d1b8d3c310e5a  guix-build-d873ff96e51a/output/arm64-apple-darwin/SHA256SUMS.part
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  65d578b81b00a1032039362dc6be1a71368f390188e0f948829afd03b8858ed2  guix-build-d873ff96e51a/output/riscv64-linux-gnu/SHA256SUMS.part
  e5233d7e7a8832893ff414c78eb3d4bca3ae30d1a1f789a23419c6739b203022  guix-build-d873ff96e51a/output/riscv64-linux-gnu/bitcoin-d873ff96e51a-riscv64-linux-gnu-debug.tar.gz
  fb6d9f5a063dc7752fcc2acc95a0052322d7c8c86d2c6373e0ceb949dcf22f49  guix-build-d873ff96e51a/output/riscv64-linux-gnu/bitcoin-d873ff96e51a-riscv64-linux-gnu.tar.gz
  61c89850244ddf5813ff80c242eff89925d30bccadfa5cb63e968c3af49eb964  guix-build-d873ff96e51a/output/x86_64-apple-darwin/SHA256SUMS.part
  cd219fab8918b061a342357d298aca0c044feb34c6d50a7851d5d3bf18cec267  guix-build-d873ff96e51a/output/x86_64-apple-darwin/bitcoin-d873ff96e51a-x86_64-apple-darwin-unsigned.dmg
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  71e93297ed8c581a7ed32a6948ef7b1ea2e7c43cb054181de3b5f604f7a2c28b  guix-build-d873ff96e51a/output/x86_64-apple-darwin/bitcoin-d873ff96e51a-x86_64-apple-darwin.tar.gz
  46e9b067ec385ee14642aebc5ec09d7d2382e0204eeb17dc64587013eddd5dff  guix-build-d873ff96e51a/output/x86_64-linux-gnu/SHA256SUMS.part
  23278b19daac51e7df65b817b79fc93562d0f4eb193ef87472456f4bed1464d7  guix-build-d873ff96e51a/output/x86_64-linux-gnu/bitcoin-d873ff96e51a-x86_64-linux-gnu-debug.tar.gz
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  eec8ab97ee9aceef8cb4e7cb5026225ffc5c7b8e8a6d376e8348020000e5af88  guix-build-d873ff96e51a/output/x86_64-w64-mingw32/SHA256SUMS.part
  a31819e67c373f30eafce8dbcb3d6d0c61d1dcf59c51023aa79321934f8a7d2a  guix-build-d873ff96e51a/output/x86_64-w64-mingw32/bitcoin-d873ff96e51a-win64-debug.zip
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  ```

ACKs for top commit:
  laanwj:
    Code review ACK d873ff96e51a3e7f2fdc3fdd1baee2bbe7583e06
  MarcoFalke:
    re-ACK d873ff96e51a3e7f2fdc3fdd1baee2bbe7583e06 only changes: 📼

Tree-SHA512: fc7d781e8cc0fc0a0080eb4b5019e91c55275e087149ed3b5abc6b691170b0ab76f1dd3ce9bb8846eef023897a89123e14751ce8facf2a170829858199904bff

1 files changed, 0 insertions, 4 deletions

diff --git a/doc/developer-notes.md b/doc/developer-notes.md
index d1b660efff..23f975df34 100644
--- a/doc/developer-notes.md
+++ b/doc/developer-notes.md
@@ -1158,10 +1158,6 @@ Current subtrees include:
 - src/crypto/ctaes
   - Upstream at https://github.com/bitcoin-core/ctaes ; maintained by Core contributors.
 
-- src/univalue
-  - Subtree at https://github.com/bitcoin-core/univalue-subtree ; maintained by Core contributors.
-  - Deviates from upstream https://github.com/jgarzik/univalue.
-
 - src/minisketch
   - Upstream at https://github.com/sipa/minisketch ; maintained by Core contributors.