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

[zgu at redhat.com]

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