RFR (XS) 8240511: Shenandoah: parallel safepoint workers count should be ParallelGCThreads

[Roman Kennke]

>> Ok yes, that makes sense.
>>
>> That flag predates upstream integration of that code, and it wasn't
>> quite clear how many threads are useful for safepoint cleanup. IIRC, I
>> found that hammering it with ParallelGCThreads was overkill - on my
>> machine. But you are right, hard-wiring it to 4 is certainly overkill on
>> smaller machines than mine ;-)
> I ran a few latency-sensitive tests on my smaller desktop, and they did not regress. I believe that
> is partly because we have trimmed down the number of parallel threads with JDK-8225229. Therefore I
> see no reason to keep it in. Another unnecessary GC option bites the dust.

Sure, go!

Roman