RFR: 8257082: ZGC: Clean up ZRuntimeWorkers and ZWorkers

[Per Liden]

On Thu, 26 Nov 2020 08:24:49 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> Found some additional stuff in ZRuntimeWorkers and ZWorkers that could be cleaned up.
>> * `ZThread::set_runtime_worker()` and friends are no longer used, and can be remove
>> * Fixed obsolete comment
>> * Added a missing `zTask.hpp` include
>> * Replaced `Monitor` with `ZConditionLock`
>
> Marked as reviewed by stefank (Reviewer).

Thanks for reviewing!

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PR: https://git.openjdk.java.net/jdk/pull/1436