Revert "Merge pull request #14544 from jefflai2/named_scope_sti"

This reverts commit 9a1abedcdeecd9464668695d4f9c1d55a2fd9332, reversing
changes made to c72d6c91a7c0c2dc81cc857a1d6db496e84e0065.

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/test/models/comment.rb

This change break integration with activerecord-deprecated_finders so
I'm reverting until we find a way to make it work with this gem.