RFR 8215299: Remove G1CMTask::should_exit_termination()'s undesirable side-effect

[zgu at redhat.com]

TerminatorTerminator::should_exit_termination() might return true to
abort ongoing termination, due to external events. However, it should
not introduce side-effect that may conflict with termination result,
because the current termination may have completed, even
TerminatorTerminator::should_exit_termination() returns true.

This undesirable side-effect is a blocker of JDK-8215047. With JDK-
8215047, termination protocol should complete in one of two consistent
state. The side-effect of setting abort flag in this particular case,
G1CMTask may re-enter termination protocol with completed terminator.


Bug: https://bugs.openjdk.java.net/browse/JDK-8215299
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8215299/webrev.00/

Test:

  hotspot_gc on Linux 64 (fastdebug and releas)

Thanks,

-Zhengyu