RFR(s): 8234086: VM operation can be simplified

[Robbin Ehn]

Hi all, please review.

CMS was the last real user of the more advantage features of VM operation.
VM operation can be simplified to always be an stack object and thus either be
of safepoint or no safepoint type.

VM_EnableBiasedLocking is executed once by watcher thread, if needed (default 
not used). Making it synchrone doesn't matter.
VM_ThreadStop is executed by a JavaThread, that thread should stop for the 
safepoint anyways, no real point in not stopping direct.
VM_ScavengeMonitors is only used to trigger a safepoint cleanup, the VM op is 
not needed. Arguably this thread should actually stop here, since we are about 
to safepoint.

There is also a small cleanup in vmThread.cpp where an unused method is removed.
And the extra safepoint is removed:
"// We want to make sure that we get to a safepoint regularly"
No we don't :)

Issue:
https://bugs.openjdk.java.net/browse/JDK-8234086
Change-set:
http://cr.openjdk.java.net/~rehn/8234086/v1/webrev/index.html

Tested scavenge manually, passes t1-2.

Thanks, Robbin