validation: Load pindexBestHeader in ChainMan

Summary:
Now BlockManager::LoadBlockIndex() will ACTUALLY only load BlockMan
members.

[META] In a later commit, pindexBestHeader will be moved to ChainMan as a member

Code Reviewer Notes

Call graph of relevant functions:

ChainstateManager::LoadBlockIndex()<-- Moved to
calls BlockManager::LoadBlockIndexDB()
which calls BlockManager::LoadBlockIndex() <-- Moved from

There is only one call to each of inner functions, meaning that no
behavior is changing.

This is a partial backport of core#24909
https://github.com/bitcoin/bitcoin/pull/24909/commits/5d670173a32ccdcb25d3a6bf97317f0ac774e0ed

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13043