Clean up and unify empty VM operations

There are a number of VM operations that do nothing, except triggering a safepoint. I'd like to clean up and unify them a bit to:
1) Use one parent class which implements the empty doit function and turns off thread oop processing.
2) Remove unused VM_Operations
3) Don't reuse the VM_None type - there's bug/inconsistency here in that some subsystems report the name as None, while others report the name passed to the constructor
4) Remove unused enum values