RFR: remove obsolete code

[Roland Westrelin]

> And that thing is actually ConP:
>
> #  assert(wb->Opcode() == Op_ShenandoahWriteBarrier) failed: expect write barrier: ConP
>
> Applying this partial reversal helps to avoid the assert:
>   http://cr.openjdk.java.net/~shade/shenandoah/revert-c2-const.patch

Right. Dropping that stuff was wrong.

> Although we probably need to follow up if we want to accept ConP in
> ShenandoahWBMemProjNode::Identity? Although I cannot see right away how it slipped through constructor:
>
>   ShenandoahWBMemProjNode(Node *src) : ProjNode( src, SWBMEMPROJCON) {
>     assert(UseShenandoahGC && ShenandoahWriteBarrier, "should be enabled");
>     assert(src->Opcode() == Op_ShenandoahWriteBarrier || src->is_Mach(), "epxect wb");
>   }

The ShenandoahWriteBarrier becomes a constant after optimizations.

Roland.