RFR: 8299701: Remove unused GCCause::_wb_conc_mark

[Kim Barrett]

Please review this change to remove GCCause::_wb_conc_mark and the supporting
implementation.  After JDK-8293824 it is no longer used as the cause when
requesting a GC.

Note: The implementation in ZGC appears to have never been used.  Prior to
JDK-8293824 this cause was only used by a G1-specific WB function.  (In the
deep mists of time it might have also been used to trigger a CMS concurrent
cycle; I didn't bother looking into that.)

Testing:
mach5 tier1

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Commit messages:
 - remove GCCause::_wb_conc_mark

Changes: https://git.openjdk.org/jdk/pull/11893/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11893&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299701
  Stats: 19 lines in 6 files changed: 0 ins; 11 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/11893.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11893/head:pull/11893

PR: https://git.openjdk.org/jdk/pull/11893