G1: Simplify HeapRegionRemSet::add_reference

HeapRegionRemSet::add_reference checks "state == Untracked" and return if true; it's called at G1RebuildRemSetClosure::do_oop_work and G1ConcurrentRefineOopClosure::do_oop_work respectively.

The check in HeapRegionRemSet::add_reference could be replaced with an assert, and move the check up to one of the caller G1RebuildRemSetClosure::do_oop_work.