runtime: remove redundant empty function call from Breakpoint on arm

CL 18964 included an extra patch (sorry, my first experience of
git-codereview) which defined the conventional breakpoint instruction
used by Plan 9 on arm, but also introduced a benign but unneeded
call to runtime.emptyfunc.  This CL removes the redundant call again.

This completes the series of CLs which add support for Plan 9 on arm.

Change-Id: Id293cfd40557c9d79b4b6cb164ed7ed49295b178
Reviewed-on: https://go-review.googlesource.com/19010Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/asm_arm.s

@@ -81,7 +81,6 @@ DATA	runtime·mainPC+0(SB)/4,$runtime·main(SB)
 GLOBL	runtime·mainPC(SB),RODATA,$4
 
 TEXT runtime·breakpoint(SB),NOSPLIT,$0-0
-	BL		runtime·emptyfunc(SB)	// force R14 save for traceback
 	// gdb won't skip this breakpoint instruction automatically,
 	// so you must manually "set $pc+=4" to skip it and continue.
 #ifdef GOOS_nacl