cmd/compile: remove formatting dreg

Left over from CL 20931.

Change-Id: I3b8dd9ef748bcbf70b5118da28135aaa1e5ba3a8
Reviewed-on: https://go-review.googlesource.com/20955
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Minux Ma <minux@golang.org>

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

@@ -529,7 +529,7 @@ func makepartialcall(fn *Node, t0 *Type, meth *Sym) *Node {
 	var fld *Node
 	var n *Node
 	for _, t := range t0.Params().Fields().Slice() {
-		n = newname(LookupN("a%d", i))
+		n = newname(LookupN("a", i))
 		i++
 		n.Class = PPARAM
 		xfunc.Func.Dcl = append(xfunc.Func.Dcl, n)