runtime: remove legacy comments and code from arm morestack

CL 137410043 deleted support for split stacks, which means morestack
no longer needed to save its caller's frame or argument size or its
caller's argument pointer. However, this commit failed to update the
comment or delete the line that computed the caller's argument
pointer. Clean these up now.

Change-Id: I65725d3d42c86e8adb6645d5aa80c305d473363d
Reviewed-on: https://go-review.googlesource.com/92437
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/vet/all/whitelist/arm.txt

@@ -2,9 +2,6 @@
 
 runtime/asm_ARCHSUFF.s: [GOARCH] cannot check cross-package assembly function: Compare is in package bytes
 
-// reflect trampolines intentionally omit arg size. Same for morestack.
-runtime/asm_arm.s: [arm] morestack: use of 4(R13) points beyond argument frame
-
 // Intentionally missing declarations.
 runtime/asm_arm.s: [arm] emptyfunc: function emptyfunc missing Go declaration
 runtime/asm_arm.s: [arm] abort: function abort missing Go declaration

src/runtime/asm_arm.s

@@ -370,10 +370,7 @@ noswitch:
  */
 
 // Called during function prolog when more stack is needed.
-// R1 frame size
 // R3 prolog's LR
-// NB. we do not save R0 because we've forced 5c to pass all arguments
-// on the stack.
 // using NOFRAME means do not save LR on stack.
 //
 // The traceback routines see morestack on a g0 as being
@@ -407,7 +404,6 @@ TEXT runtime·morestack(SB),NOSPLIT|NOFRAME,$0-0
 	// Set m->morebuf to f's caller.
 	MOVW	R3, (m_morebuf+gobuf_pc)(R8)	// f's caller's PC
 	MOVW	R13, (m_morebuf+gobuf_sp)(R8)	// f's caller's SP
-	MOVW	$4(R13), R3			// f's argument pointer
 	MOVW	g, (m_morebuf+gobuf_g)(R8)
 
 	// Call newstack on m->g0's stack.