G1DirtyCardQueueSet _notify_when_complete is always true

G1DirtyCardQueueSet has a _notify_when_complete member, used to control whether adding completed buffers should notify the completed buffer monitor. It is initialized from a constructor argument that defaults to true.

With the separation of G1RedirtyCardsQueueSet from G1DirtyCardQueueSet (JDK-8162929), the DCQS is a singleton, with that default value never overridden and the member never modified. We can just remove the member and conditionalization on it.