vendor: switch to using go1.11 modules

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

@@ -1,13 +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.
-
-// +build !darwin,!dragonfly,!freebsd,!linux,!netbsd,!openbsd,!solaris,!windows
-
-package ipv4
-
-import "golang.org/x/net/internal/socket"
-
-func setControlMessage(c *socket.Conn, opt *rawOpt, cf ControlFlags, on bool) error {
-	return errOpNoSupport
-}