Drop feature test for dlopen()

With libtool being mandatory from 9.17 on, so is dlopen() (via libltdl).

bin/tests/system/dlzexternal/prereq.sh

@@ -1,18 +0,0 @@
-#!/bin/sh
-#
-# Copyright (C) Internet Systems Consortium, Inc. ("ISC")
-#
-# 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 http://mozilla.org/MPL/2.0/.
-#
-# See the COPYRIGHT file distributed with this work for additional
-# information regarding copyright ownership.
-
-. $SYSTEMTESTTOP/conf.sh
-
-$FEATURETEST --have-dlopen ||  {
-        echo_i "dlopen() not supported - skipping dlzexternal test"
-        exit 255
-}
-exit 0