validation: replace lock with annotation in UnloadBlockIndex()

Summary:
This is a partial backport of core#24299
https://github.com/bitcoin/bitcoin/pull/24299/commits/daad0093e3d1466789d0ce687902636c80cd74a1

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12988