RFR: 8079080: ConcurrentMark::mark_stack_push(oop) is unused

[Kim Barrett]

Please review this cleanup of ConcurrentMark and CMMarkStack.

The following functions were unused, so are being removed:

- ConcurrentMark::mark_stack_push(oop) - the other overload is used and retained.
- CMMarkStack::push(oop)
- CMMarkStack::par_push(oop)
- CMMarkStack::par_adjoin_arr(oop*, int)

In addition, CMMarkStack::pop() is only used in the implementation of
CMMarkStack::drain(), so is being made private.

CR:
https://bugs.openjdk.java.net/browse/JDK-8079080

Webrev:
http://cr.openjdk.java.net/~kbarrett/8079080/webrev.00/

Testing:
JPRT