RFR: Remove ShenandoahMarkCompactBarrierSet

[Roman Kennke]

This came out of the upstream merge that I'm working on. Upstream
doesn't allow to re-set the BarrierSet and I think this is good.
However, we are temporarily re-setting the BS during mark-compact to
prevent accidentally calling into RB and resolving garbage (because
fwd-ptr is not always pointing to valid object during adjust-ptrs phase).

I am proposing to revert the hack and remove ShMCBS and instead check
has_forwarded_objects() in the RB. And of course, un-setting
has_forwarded_objects after mark-compact-phase1-marking. I would argue
that this is reasonable because the fwd ptr is not guaranteed to point
to anything meaningful anyway.

http://cr.openjdk.java.net/~rkennke/mark-compact-bs/webrev.00/

Testing: hotspot_gc_shenandoah

Ok?

Thanks,
Roman