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

[Per Liden]

On Thu, 9 Sep 2021 13:42:29 GMT, Per Liden <pliden at openjdk.org> wrote:

>> 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

Thanks all for reviewing!

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