refactor: remove unused assumeutxo methods

Summary:
After feedback from Russ, I realized that there are some extraneous assumeutxo methods
that are not necessary and probably just overly confusing. These include

• Validated*()
• IsBackgroundIBD()

and they can be removed.

Also add comment for g_best_block

This is a backport of core#21526 [2 & 3/12]
https://github.com/bitcoin/bitcoin/pull/21526/commits/ac4051d891e2d5c8ac130da16b85b9d880b44720
https://github.com/bitcoin/bitcoin/pull/21526/commits/665072a36df2e4c88705fedd4ac7c955d7f6a488

Backported out of order (unrelated refactorings first)

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12267