RFR (S): 8200074: Remove G1ConcurrentMark::_concurrent_marking_in_progress

[Thomas Schatzl]

Hi all,

  can I have reviews for the following change that removes
G1ConcurrentMark::_concurrent_marking_in_progress since it is only used
in some asserts, and at the same time mostly the same
as G1ConcurrentMark::_concurrent (ie. it is used to verify
_concurrent).

CR:
https://bugs.openjdk.java.net/browse/JDK-8200074
Webrev:
http://cr.openjdk.java.net/~tschatzl/8200074/webrev
Testing:
hs-tier 1-5 with other changes

Change needs JDK-8200255 to apply cleanly.

Thanks,
  Thomas