internal/syscall/unix: remove unnecessary empty.s

After CL 130736 there are no empty function declarations this package
anymore, so empty.s is no longer needed.

Change-Id: Ic4306f10ad8a31777a3337870ce19e14c1510f3b
Reviewed-on: https://go-review.googlesource.com/131835
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/internal/syscall/unix/empty.s

-// Copyright 2018 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// This file is here just to make the go tool happy. It allows
-// empty function declarations (no function body).
-// It is used with "go:linkname".