cmd/internal/obj/arm: delete unnecessary code

In the arm assembler, "AMOVW" never falls into optab
case 13, so the check "if p.As == AMOVW" is useless.

Change-Id: Iec241d5b4cffb358a1477f470619dc9a6287884a
Reviewed-on: https://go-review.googlesource.com/c/138575
Run-TryBot: Ben Shi <powerman1st@163.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/internal/obj/arm/asm5.go

@@ -2007,7 +2007,7 @@ func (c *ctxt5) asmout(p *obj.Prog, o *Optab, out []uint32) {
 		o2 = c.oprrr(p, p.As, int(p.Scond))
 		o2 |= REGTMP & 15
 		r := int(p.Reg)
-		if p.As == AMOVW || p.As == AMVN {
+		if p.As == AMVN {
 			r = 0
 		} else if r == 0 {
 			r = int(p.To.Reg)