RFR 8215549: Shenandoah deduplication cleans up table/queue twice

[zgu at redhat.com]

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