Remove resolve and obj_equals leftovers from BarrierSetAssembler

Shenandoah used to require resolve() and obj_equals() in its first incarnation in JDK12. This is long obsolete, and the corresponding code can be removed from BarrierSetAssembler. It also makes the code less confusing because obj_equals() and resolve() have both been rather clunky and given that they now do nothing (and there are other resolve* methods in BSA too), it's not even clear what they would be supposed to do. It also makes life easier for porters to other platforms.