RFR: Unquarantine ShenandoahStrDedupStress test

[Zhengyu Gu]

Hi,

I think we have resolved timeout and negative index OOB issues for 
ShenandoahStrDedupStress test.

Aleksey had 10 runs of tier3_gc_shenandoah test, and I had 6 runs (both 
release and fastdebug), all clean.

Let's remove it from ProblemList

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/unquarantine_dedup_stress/webrev.00/


Thanks,

-Zhengyu