Remove vendor directory and add it to the ignore list

vendor/golang.org/x/sys/windows/env_unset.go

@@ -1,15 +0,0 @@
-// 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.
-
-// +build windows
-// +build go1.4
-
-package windows
-
-import "syscall"
-
-func Unsetenv(key string) error {
-	// This was added in Go 1.4.
-	return syscall.Unsetenv(key)
-}