Remove KCP (fixes #4737) (#4741)

lib/connections/deprecated.go

@@ -0,0 +1,36 @@
+// Copyright (C) 2018 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package connections
+
+import "github.com/syncthing/syncthing/lib/config"
+
+// deprecatedListener is never valid
+type deprecatedListener struct {
+	listenerFactory
+}
+
+func (deprecatedListener) Valid(_ config.Configuration) error {
+	return errDeprecated
+}
+
+// deprecatedDialer is never valid
+type deprecatedDialer struct {
+	dialerFactory
+}
+
+func (deprecatedDialer) Valid(_ config.Configuration) error {
+	return errDeprecated
+}
+
+func init() {
+	listeners["kcp"] = deprecatedListener{}
+	listeners["kcp4"] = deprecatedListener{}
+	listeners["kcp6"] = deprecatedListener{}
+	dialers["kcp"] = deprecatedDialer{}
+	dialers["kcp4"] = deprecatedDialer{}
+	dialers["kcp6"] = deprecatedDialer{}
+}