Move blockstorage-related unload to BlockManager::Unload
Summary:
This is a refactor and safe to do because:
- UnloadBlockIndex calls ChainstateManager::Unload, which calls BlockManager::Unload
- Only unit tests call Unload directly
This is a partial backport of core#23974
https://github.com/bitcoin/bitcoin/pull/23974/commits/fab262174b96854d2df5bee7da578990c9e9cb1e
Depends on D12511
Test Plan: ninja all check-all
Reviewers: #bitcoin_abc, sdulfari
Reviewed By: #bitcoin_abc, sdulfari
Differential Revision: https://reviews.bitcoinabc.org/D12512