RFR (S) 8225743: Remove ShenandoahBarrierSetC2::escape_is_barrier_node and related GC interface

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8225743

This is the leftover from the times Shenandoah had an option to not optimize out barriers on newly
allocated objects. This is have not been needed for a while. Since Shenandoah is the only user of
this method, it makes sense to remove it completely,

Fix:
  http://cr.openjdk.java.net/~shade/8225743/webrev.01/

Testing: hotspot_gc_shenandoah, jdk-submit

-- 
Thanks,
-Aleksey