Make BlockManager::LoadBlockIndex private

Summary:
BlockManager::LoadBlockIndex is only called by BlockManager::LoadBlockIndexDB. Thus, it can be made private.
m_best_invalid is no longer accessed by BlockManager::LoadBlockIndex. Thus, the unused friend can be removed.

This is a backport of core#24917

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13070