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>