[sh/8u] RFR Shenandoah: Backed out weak roots cleaning during full gc

[Zhengyu Gu]

This patch backout the workaround [1] for Cassandra failure.
The root cause was fixed by JDK-8248041 [2] and backported in sh/jdk8u, 
therefore, this workaround is no longer needed.

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/sh-8u-backout-weak-roots-cleaning-fullgc/webrev.00/

Test:
   hotspot_gc_shenandoah.

Thanks,

-Zhengyu


[1] 
https://cr.openjdk.java.net/~shade/shenandoah/8u-fullgc-jni-weak/webrev.01/
[2] https://bugs.openjdk.java.net/browse/JDK-8248041