RFR: 8273545: Remove Thread::is_GC_task_thread()

[Per Liden]

I propose we remove Thread::is_GC_task_thread(). It's used only in two places (one in ZGC, and one assert in ParallelGC), and those two uses can be replaced by calls to is_Worker_thread() instead. Removing is_GC_task_thread() also allows us to clean out some stuff from WorkGang/GangWorker.

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Commit messages:
 - 8273545: Remove Thread::is_GC_task_thread()

Changes: https://git.openjdk.java.net/jdk/pull/5442/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5442&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273545
  Stats: 22 lines in 12 files changed: 0 ins; 16 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5442.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5442/head:pull/5442

PR: https://git.openjdk.java.net/jdk/pull/5442