RFR(T): 8230332: G1DirtyCardQueueSet _notify_when_complete is always true

[Kim Barrett]

Please review this trivial cleanup of G1DirtyCardQueueSet.  With the
separation of G1RedirtyCardsQueueSet from G1DirtyCardQueueSet, the
latter is now a singleton class.  As a result, the
_notify_when_complete member (used to control whether adding completed
buffers should notify the completed buffer monitor) is always true.

This change removes that member and changes the conditional
notifications to be unconditional.  Also cleaned up some locker usage
for _cbl_mon when notification is needed, and changed to consistently
use notify_all() (there's no good reason to use notify(), and
definitely no good reason to use a mix of the two here).

CR:
https://bugs.openjdk.java.net/browse/JDK-8230332

Webrev:
http://cr.openjdk.java.net/~kbarrett/8230332/open.00/

Testing:
Local (linux-x64) hotspot:tier1