RFR: Remove ShFastSyncRoots

[Roman Kennke]

We have a fast synchronizer roots iterator, but it's not really used. It
used to be useful when we did not have thread-local MonitorInUseLists
and needed to scan one global list efficiently, but MonitorInUseLists
provides much more natural parallelization via thread stacks. Also,
MonitorInUseLists is deprecated and I see no point why anybody would
disable it. Let's remove the extra cruft:

http://cr.openjdk.java.net/~rkennke/remove-fast-syncroots/webrev.00/

Testing: tier3_gc_shenandoah

Ok?

Roman