FRF(S) 8216072: Remove TaskTerminator's assignment operator

[Roman Kennke]

Looks good to me!

Thanks,
Roman

> The assignment operator was a workaround for issues since resolved by
> JDK-8215047, JDK-8215299 and JDK-8215220. It is time to remove it and
> use ParallelTaskTerminator::reset_for_reuse() instead.
> 
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8216072
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8216072/webrev.00/index.htm
> l
> 
> 
> Test:
> 
>    hotspot_gc (+/-UseOWSTTaskTerminator) on Linux x64 (fastdebug and
> release)
> 
> Thanks,
> 
> -Zhengyu
>