RFR 8215549: Shenandoah deduplication cleans up table/queue twice

[Roman Kennke]

Hi Zhengyu,

this looks good to me.

Thanks,
Roman

> Shenandoah string deduplication cleans up deduplication table and
> queues twice, once via ParallelCleaningTask and then
> ShenandoahStringDedup::parallel_cleanup(). The second cleanup should be
> removed after we adopted upstream deduplication implementation.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8215549
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8215549/webrev.00/
> 
> Test:
> 
>   tier1,2,3 gc_shenandoah on Linux 64 (fastdebug and release)
> 
> 
> Thanks,
> 
> -Zhengyu
>