[15] RFR(T) 8237017: Shenandoah: Remove racy assertion

[Zhengyu Gu]

During concurrent weak root processing, it tries to CAS in NULL if the 
oop is dead, then asserts that the slot is indeed NULL.

The assertion is racy, because there can be another thread to release 
the slot and then reuse it (that's why it uses CAS in the first place), 
that can result assertion to fail.

Bug: https://bugs.openjdk.java.net/browse/JDK-8237017
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8237017/webrev.00/index.html

Test:
   hotspot_gc_shenandoah fastdebug on x86_64 Linux

Thanks,

-Zhengyu