os: remove superfluous comments

It's apparent from the file names or build tags to which OS the code in
question applies.

Change-Id: I628ee2bf1d29a6bc30ca5fa6f9eecf809e78a182
Reviewed-on: https://go-review.googlesource.com/110815
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/sys_bsd.go

@@ -4,9 +4,6 @@
 
 // +build darwin dragonfly freebsd nacl netbsd openbsd
 
-// os code shared between *BSD systems including OS X (Darwin)
-// and FreeBSD.
-
 package os
 
 import "syscall"

src/os/sys_linux.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Linux-specific
-
 package os
 
 import (

src/os/sys_plan9.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Plan 9-specific
-
 package os
 
 func hostname() (name string, err error) {