validation: Move PruneOneBlockFile to BlockManager

Summary:
[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

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9822