RFR: Remove workaround for JDK-8205006

[Zhengyu Gu]

Cherry pick upstream fix for JDK-8205006, so to remove the workaround.

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/rm-workaround-8205006/webrev.00/

Test:

   tier3_gc_shenandoah.

Thanks,

-Zhengyu