Before JDK-8343782 G1 iterated remsets for humongous eager reclaim candidates by region, as there has been a 1:1 mapping between regions and (card-based) remembered sets.

With that change, all remembered set iteration should go via the group remsets (`G1CSetCandidateGroup`), and the code does, except for humongous eager reclaim candidates which handle their group remsets on the side (which is a separate issue).

This obsoleted some code in `G1MergeCardSetClosure` - it does not need to be a `G1HeapRegionClosure` any more, and for humongous regions we can directly do the iteration via the group rem set attached to the humongous starts region. I.e. inline that `merge_card_set_for_region()` method.