cmd/compile: remove unused writebarrierptr, typedmemmove Nodes

Now that SSA's write barrier pass is generating calls to these,
compile doesn't need to look them up.

Change-Id: Ib50e5f2c67b247ca280d467c399e23877988bc12
Reviewed-on: https://go-review.googlesource.com/32170
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/compile/internal/gc/go.go

@@ -380,7 +380,4 @@ var panicdivide *Node
 
 var growslice *Node
 
-var writebarrierptr *Node
-var typedmemmove *Node
-
 var panicdottype *Node

src/cmd/compile/internal/gc/pgen.go

@@ -303,8 +303,6 @@ func compile(fn *Node) {
 		panicslice = Sysfunc("panicslice")
 		panicdivide = Sysfunc("panicdivide")
 		growslice = Sysfunc("growslice")
-		writebarrierptr = Sysfunc("writebarrierptr")
-		typedmemmove = Sysfunc("typedmemmove")
 		panicdottype = Sysfunc("panicdottype")
 	}