[15] RFR 8243578: Shenandoah: Cleanup ShenandoahStringDedup::parallel_oops_do()

[Zhengyu Gu]

Please review this small cleanup. It removes a branch that works around 
the problem for traversal GC.

Bug: https://bugs.openjdk.java.net/browse/JDK-8243578
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8243578/webrev.00/

Test:
  hotspot_gc_shenandoah

-Zhengyu