unix: remove duplicate build tag

Remove a duplicate build tag in unix/flock.go. Keep the alphabetically
sorted version below the copyright header.

Change-Id: Ie3a6e5e01e5e4114960fdf63111a68d3b2b8de47
Reviewed-on: https://go-review.googlesource.com/51051Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/flock.go

-// +build linux darwin freebsd openbsd netbsd dragonfly
-
 // Copyright 2014 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.