[15] RFR(T) 8237874: Shenandoah: Backout JDK-8234399

[Zhengyu Gu]

I would like to backout JDK-8234399, as Shenandoah still triggers 
barriers on GC paths.

Bug: https://bugs.openjdk.java.net/browse/JDK-8237874
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8237874/webrev/

Test:
   hotspot_gc_shenandoah and test case in bug report.


Thanks,

-Zhengyu