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

This reverts commit 807ee0ac67fe094a4439fdf616c5a5bdbb3bd050.

Conflicts:
	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.