[META] This is a pure refactor commit.
Move PruneBlockFile to BlockManager because:
1. PruneOneBlockFile only acts on BlockManager
2. Eliminates the need for callers (FindFilesToPrune{,Manual}) to have a
reference to the larger ChainstateManager, just a reference to
BlockManager is enough. See following commits.
This is a backport of core#19927 [3/6]
https://github.com/bitcoin/bitcoin/pull/19927/commits/f8d4975ab3fcd3553843cf0862251289c88c106b
Depends on D9821