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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  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
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  5955d28e6d56e5a3297dab723b8478f1b0bb7f5b86476c581339122f34cc7f14  guix-build-d873ff96e51a/output/arm-linux-gnueabihf/bitcoin-d873ff96e51a-arm-linux-gnueabihf-debug.tar.gz
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  ```

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

Tree-SHA512: fc7d781e8cc0fc0a0080eb4b5019e91c55275e087149ed3b5abc6b691170b0ab76f1dd3ce9bb8846eef023897a89123e14751ce8facf2a170829858199904bff

0 files changed, 0 insertions, 0 deletions