Make collection set regions not need remembered set update during GC

By making collection set regions (not candidates!) mark as not needing remembered set update, some checks in the calculations whether a reference should be enqueued can be simplified after JDK-8271880 as suggested by [~ayang].

Initial tests show this is correct because either
- the collection set region will be empty after gc and collected
- the collection set region will have failed evacuation but g1 currently never builds remembered sets for these regions

Do some more testing on this too.