RFR: Remove Parallel Cleanup counters

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/purge-purge-counters/webrev.01/

We have switched to upstream version of parallel cleanup, which does not have our hooks into the GC
counters -- which is why the block is commented out. While we can upstream the API extensions to get
that data, it seems more convenient to accept those counters are not really needed, and purge the
our own counter infra.

Testing: tier3_gc_shenandoah

Thanks,
-Aleksey