RFR: 8299701: Remove unused GCCause::_wb_conc_mark [v2]

[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

Kim Barrett has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge branch 'master' into wb_conc_mark
 - remove GCCause::_wb_conc_mark

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/11893/files
  - new: https://git.openjdk.org/jdk/pull/11893/files/d4cefe03..f1aeb03d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11893&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11893&range=00-01

  Stats: 4200 lines in 157 files changed: 2391 ins; 1080 del; 729 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