RFR: 8273545: Remove Thread::is_GC_task_thread() [v2]

[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.

Per Liden has updated the pull request incrementally with one additional commit since the last revision:

  Updated gtests

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5442/files
  - new: https://git.openjdk.java.net/jdk/pull/5442/files/ca94d767..0165c4a9

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5442&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5442&range=00-01

  Stats: 6 lines in 5 files changed: 0 ins; 2 del; 4 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