RFR(T) 8221875: Unquarantine Shenandoah string dedup tests

[Zhengyu Gu]

The cause of failures are fixed by JDK-8220671 and JDK-8221102, let's 
put the tests back on.

Bug: https://bugs.openjdk.java.net/browse/JDK-8221875


diff -r 9559ba212c18 -r 5aacfca3a4f1 test/hotspot/jtreg/ProblemList.txt
--- a/test/hotspot/jtreg/ProblemList.txt        Tue Apr 02 13:08:38 2019 
-0400
+++ b/test/hotspot/jtreg/ProblemList.txt        Tue Apr 02 16:30:45 2019 
-0400
@@ -141,9 +141,6 @@

  # :hotspot_gc_shenandoah

-gc/shenandoah/TestStringDedup.java 
8220671,8221102 generic-all
-gc/shenandoah/TestStringDedupStress.java 
8220671,8221102 generic-all
-
 
#############################################################################

  # :hotspot_misc


Thanks,

-Zhengyu