add prune and remove unused packages

vendor/golang.org/x/sys/plan9/pwd_plan9.go

@@ -1,23 +0,0 @@
-// Copyright 2015 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.
-
-// +build !go1.5
-
-package plan9
-
-func fixwd() {
-}
-
-func Getwd() (wd string, err error) {
-	fd, err := open(".", O_RDONLY)
-	if err != nil {
-		return "", err
-	}
-	defer Close(fd)
-	return Fd2path(fd)
-}
-
-func Chdir(path string) error {
-	return chdir(path)
-}