Completely remove the Unix Domain Socket support from BIND 9

The Unix Domain Sockets support in BIND 9 has been completely disabled
since BIND 9.18 and it has been a fatal error since then.  Cleanup the
code and the documentation that suggest that Unix Domain Sockets are
supported.

bin/tests/system/checkconf/bad-unix-domain-socket.conf

@@ -0,0 +1,16 @@
+/*
+ * Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+ *
+ * SPDX-License-Identifier: MPL-2.0
+ *
+ * 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/.
+ *
+ * See the COPYRIGHT file distributed with this work for additional
+ * information regarding copyright ownership.
+ */
+
+controls {
+	unix "/tmp/socket" perm 0700 owner 0 group 0;
+};