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

[Zhengyu Gu]

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