RFR (S): 8200255: Remove G1CMTask::_concurrent

[Thomas Schatzl]

Hi all,

  can I have reviews for this small change that removes
G1CMTask::_concurrent. That member is a duplicate of
G1ConcurrentMark::_concurrent, but there does not seem to be any
advantage of keeping this duplicate, as it's only used in asserts or
non-performance critical code.

This removes some imho unnecessary code, so I suggest to do that.

CR:
https://bugs.openjdk.java.net/browse/JDK-8200255
Webrev:
http://cr.openjdk.java.net/~tschatzl/8200255/webrev/
Testing:
local compilation, gcbasher, testing with other changes

Thanks,
  Thomas