Fix outdated comments referring to ::ChainActive()

Summary:
Remove also a comment about g_chainstate.
The additional ::ChainActive comment removed in validation.cpp was missed in a backport in D1972.

This is a backport of core#23258

Test Plan:

grep -r "::Chain\w*A" ../src/
grep -r "g_chainman" ../src/

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11755