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

[Roman Kennke]

Yes please remove that.

Thanks,
Roman

> 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
>