RFR(s): 8236766: Remove un-used oops do and drain list in VM thread.

[Robbin Ehn]

Hi all, please review.

We have removed all oops and oops do from the VM operations.
We can now remove the oops do on VM op base class, oops do on VM queue and the 
drain list.

Note that I also removed the unused VMOperationQueue::verify_queue.

Issue:
https://bugs.openjdk.java.net/browse/JDK-8236766

Changeset:
http://cr.openjdk.java.net/~rehn/8236766/v1/webrev/index.html

Passes t1-3.

Thanks, Robbin