RFR: 8047819: G1 HeapRegionDCTOC does not need to inherit ContiguousSpaceDCTOC

[Mikael Gerdin]

Hi!

As part of a larger effort to detach G1's HeapRegion from ContiguousSpace[1]
G1 needs to stop using the special version of DirtyCardToOopClosure.
This also makes the code more easy to follow since G1 never actually 
relies on the functionality from Filtering_DCTOC and ContiguousSpaceDCTOC.

This change is part of a set of 4 changes: 8047818, 8047819, 8047820, 8047821 
which are needed to refactor the HeapRegion class and its superclasses 
in order to simplify the G1 class unloading change which is coming.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8047819
Webrev:
http://cr.openjdk.java.net/~mgerdin/8047819/webrev/

[1] https://bugs.openjdk.java.net/browse/JDK-8047818

Thanks
/Mikael