vendor: switch to using go1.11 modules

vendor/golang.org/x/net/ipv4/control_windows.go

@@ -1,16 +0,0 @@
-// Copyright 2012 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.
-
-package ipv4
-
-import (
-	"syscall"
-
-	"golang.org/x/net/internal/socket"
-)
-
-func setControlMessage(c *socket.Conn, opt *rawOpt, cf ControlFlags, on bool) error {
-	// TODO(mikio): implement this
-	return syscall.EWINDOWS
-}