RFR (sh/jdk11): Remove shared-code changes in callnode.cpp|hpp

[Roman Kennke]

The CallLeafNode::Ideal() has been added in order to let Shenandoah
optimize leaf nodes. However, sh/jdk11 also already has the
BarrierSetC2::ideal_node() machinery in phaseX.cpp. Which means this is
not needed and in-fact probably calls into
ShenandoahBarrierSetC2::ideal_node() 2x for no reason. Let's remove this.

Webrev:
http://cr.openjdk.java.net/~rkennke/shjdk11-cleanup-callnode/webrev.00/
Testing: hotspot_gc_shenandoah

Ok to push?

Roman