7143490: G1: Remove HeapRegion::_top_at_conc_mark_count Summary: Removed the HeapRegion::_top_at_conc_mark_count field. It is no longer needed as a result of the changes for 6888336 and 7127706. Refactored the closures that finalize and verify the liveness counting data so that common functionality was placed into a base class. Reviewed-by: brutisso, tonyp