cmd/compile: remove Node.Nname, now unused

$ sizeof -p cmd/compile/internal/gc Node
Node 160
$

Change-Id: Ib0bd4230c8547f36972b2a9d81ba3eca81496e39
Reviewed-on: https://go-review.googlesource.com/10537Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -21,9 +21,8 @@ type Node struct {
 	Rlist *NodeList
 
 	// most nodes
-	Type  *Type
-	Orig  *Node // original form, for printing, and tracking copies of ONAMEs
-	Nname *Node
+	Type *Type
+	Orig *Node // original form, for printing, and tracking copies of ONAMEs
 
 	// func
 	Func *Func