cmd/internal/obj/arm, runtime: delete old ARM softfloat code

cmd/internal/obj/arm, runtime: delete old ARM softfloat code

CL 106735 changed to the new softfloat support on GOARM=5.

ARM assembly code that uses FP instructions not guarded on GOARM,
if any, will break. The easiest way to fix is probably to use Go
implementation on GOARM=5, like

	MOVB	runtime·goarm(SB), R11
	CMP	$5, R11
	BEQ	arm5
	... FP instructions ...
	RET
arm5:
	CALL or JMP to Go implementation

Change-Id: I52fc76fac9c854ebe7c6c856c365fba35d3f560a
Reviewed-on: https://go-review.googlesource.com/107475
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>