RFR: 8149793: DirtyCardQueueSet::apply_closure_to_completed_buffer_helper isn't helpful

[Kim Barrett]

Please review this small cleanup, merging a non-helpful "helper" into
its only caller.

CR:
https://bugs.openjdk.java.net/browse/JDK-8149793

Webrev:
http://cr.openjdk.java.net/~kbarrett/8149793/webrev.00/

Testing:
JPRT