playbooks/roles/exim: remove "true" filter plugin

This filter is unsued in the role, remove it.

This allows it to be run under zuul and can be moved into the
top-level role/ directory later.

Change-Id: Ice97f0c3c9f52b6bf9f48c7b16d577e555924034

playbooks/roles/exim/filter_plugins/filters.py

@@ -1,24 +0,0 @@
-# Copyright 2018 Red Hat, Inc.
-#
-# Licensed under the Apache License, Version 2.0 (the "License"); you may
-# not use this file except in compliance with the License. You may obtain
-# a copy of the License at
-#
-#      http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
-# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
-# License for the specific language governing permissions and limitations
-# under the License.
-
-
-def true_filter(v):
-    return v is True
-
-class FilterModule(object):
-
-    def filters(self):
-        return {
-            'true': true_filter
-        }