RFR [8]: Cleanup undeclared methods in barrier stubs

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/8u-stubs-cleanup/webrev.01/

This is 8u-specific. Andrew Hughes found some of the methods in our stubs are not actually declared
in ShenandoahBarrierSet. Let's purge them.

Testing: zero build

Thanks,
-Aleksey