RFR: 8359924: Deprecate and obsolete ParallelRefProcEnabled

[Kim Barrett]

On Wed, 18 Jun 2025 14:04:28 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Deprecating `ParallelRefProcEnabled`, which is used only by Parallel and G1, and both have it enabled by default via:
> 
> 
>   if (FLAG_IS_DEFAULT(ParallelRefProcEnabled) && ParallelGCThreads > 1) {
>     FLAG_SET_DEFAULT(ParallelRefProcEnabled, true);
>   }
> 
> 
> Disabling it offers little benefit and its presence incurs some implementation complexity in the reference-processor.

Looks good.

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Marked as reviewed by kbarrett (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/25875#pullrequestreview-2940716200