RFR: Remove Parallel Cleanup counters

[Roman Kennke]

This makes sense. When upstream comes up with its own counters
infrastructure, we'll re-introduce whatever is needed.

Thanks.
Roman

> 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
>