Remove obsolete null-handling block in ShenandoahBarrierNode::skip_through_barrier()

There is a block that attempts to skip a Shenandoah barrier that is wrapped in a null-check. This appears to be a left-over from when we expanded barriers in .ad. We should have no business dealing with null-checks that we haven't generated ourselves.

This affects a bug-fix that we need to do for shenandoah/jdk11, where we seem to be break the graph when keeping that code.