RFR(12u): 8223144: Remove obsolete null-handling block in ShenandoahBarrierNode::skip_through_barrier()

[Roman Kennke]

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.

The block is already gone in jdk/jdk.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8223144
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8223144/webrev.00/

Testing: hotspot_gc_shenandoah passes

Roman