Parallel: Remove VMThread specific ParCompactionManager

For some reason Parallel GC allocates ParallelGCThreads+1 ParCompactionManager, the last one only using for work in the VM thread. That ParCompactionManager`s task queue is also not wired into the work stealing mechanism (i.e. the TaskQueueSet), causing unnecessary code duplication when printing these statistics.

Remove that unnecessarily allocated task queue as it also prevents factoring out task queue stats code, and use the 0'th ParcompactionManager for these tasks (which the documentation states to be just fine).