RFR: 8201362: Remove CollectedHeap::barrier_set()

[Per Liden]

This patch removes the redundant CollectedHeap::_barrier_set and the
setter/getter. Throughout the code, we currently have a mix of calls to 
BarrierSet::barrier_set() and Universe::heap()->barrier_set(). This 
patch unifies all that to always use BarrierSet::barrier_set().

Bug: https://bugs.openjdk.java.net/browse/JDK-8201362
Webrev: http://cr.openjdk.java.net/~pliden/8201362/webrev.0

/Per