Merge #19927: validation: Reduce direct g_chainman usage

72a1d5c6f3834e206719ee5121df7727aed5b786 validation: Remove review-only comments + assertions (Carl Dong)
3756853b15902d63f4b5a3129e8b5d82e84e125b docs: Move FindFilesToPrune{,Manual} doxygen comment (Carl Dong)
485899a93c6f5fff62090907efb0ac938992e1fb style: Make FindFilesToPrune{,Manual} match style guide (Carl Dong)
3f5b5f3f6db0e5716911b3fba1460ce327e8a845 validation: Move FindFilesToPrune{,Manual} to BlockManager (Carl Dong)
f8d4975ab3fcd3553843cf0862251289c88c106b validation: Move PruneOneBlockFile to BlockManager (Carl Dong)
74f73c783d46b012f375d819e2cd09c792820cd5 validation: Pass in chainman to UnloadBlockIndex (Carl Dong)
4668ded6d6ea4299d998abbb57543f37519812e2 validation: Move ~CMainCleanup logic to ~BlockManager (Carl Dong)

Pull request description:

  This PR paves the way for de-globalizing `g_chainman` entirely by removing the usage of `g_chainman` in the following functions/methods:
  - `~CMainCleanup`
  - `CChainState::FlushStateToDisk`
  - `UnloadBlockIndex`

  The remaining direct uses of `g_chainman` are as follows:
  1. In initialization codepaths:
  	- `AppTests`
  	- `AppInitMain`
  	- `TestingSetup::TestingSetup`
  2. `::ChainstateActive`
  3. `LookupBlockIndex`
  	- Note: `LookupBlockIndex` is used extensively throughout the codebase and require a much larger set of changes, therefore I've left it out of this initial PR

ACKs for top commit:
  MarcoFalke:
    re-ACK 72a1d5c6f3 👚
  jnewbery:
    utACK 72a1d5c6f3834e206719ee5121df7727aed5b786

Tree-SHA512: 944a4fa8405eecf39706ff944375d6824373aaeea849d11473f08181eff26b12f70043a8348a5b08e6e9021b243b481842fbdfbc7c3140ca795fce3688b7f5c3

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