TUN-813: Clean up cloudflared dependencies

vendor/golang.org/x/sys/plan9/env_plan9.go

@@ -1,31 +0,0 @@
-// Copyright 2011 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.
-
-// Plan 9 environment variables.
-
-package plan9
-
-import (
-	"syscall"
-)
-
-func Getenv(key string) (value string, found bool) {
-	return syscall.Getenv(key)
-}
-
-func Setenv(key, value string) error {
-	return syscall.Setenv(key, value)
-}
-
-func Clearenv() {
-	syscall.Clearenv()
-}
-
-func Environ() []string {
-	return syscall.Environ()
-}
-
-func Unsetenv(key string) error {
-	return syscall.Unsetenv(key)
-}