~CMainCleanup:
1. Is vestigial
2. References the g_chainman global (we should minimize g_chainman refs)
3. Only acts on g_chainman.m_blockman
4. Does the same thing as BlockManager::Unload
This is a backport of core#19927 [1/6]
https://github.com/bitcoin/bitcoin/pull/19927/commits/4668ded6d6ea4299d998abbb57543f37519812e2
Backport note: there were 7 commits in the original PR, but one of those commits is only touching style and is not relevant to us as the linter already fixed the style.