os: remove duplicate package description

file.go contains a more complete package description.

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/4645058

src/pkg/os/file_posix.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.
 
-// The os package provides a platform-independent interface to operating
-// system functionality.  The design is Unix-like.
 package os
 
 import (