RFR(s): 8197993: Remove last use of JavaThread::flush_barrier_queues()

[Per Liden]

With the introduction of BarrierSet::flush_deferred_barriers() there's 
no reason to keep JavaThread::flush_barrier_queues() around anymore. 
Replace the last call-site with a call to the barrier set instead.

Bug: https://bugs.openjdk.java.net/browse/JDK-8197993
Webrev: http://cr.openjdk.java.net/~pliden/8197993/webrev.0/

/Per