RFR(S) 8225813: Shenandoah: Obsolete ShenandoahSuspendibleWorkers flag, as suspendible workers are required

[Roman Kennke]

Can you give a little bit of explanation why workers need to be
suspended at safepoints for concurrent root processing?

Other than that, the patch looks good.

Roman

> Please review this patch that makes suspendible workers required,
> instead of optional, and obsoletes the corresponding flag.
> 
> Suspendible workers are required for concurrent root processing.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8225813
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8225813/webrev.00/index.html
> 
> Test:
>   hotspot_gc_shenandoah (fastdebug and release)
> 
> Thanks,
> 
> -Zhengyu