Undo the deletion of some tables (#6047)

This is a partial revert of #5893. The problem is that if we drop these tables
in the same release as removing the code that writes to them, it prevents users
users from being able to roll back to a previous release.

So let's leave the tables in place for now, and remember to drop them in a
subsequent release.

(Note that these tables haven't been *read* for *years*, so any missing rows
resulting from a temporary upgrade to vNext won't cause a problem.)

changelog.d/5893.misc

@@ -1 +1 @@
-Drop some unused tables.
+Stop populating some unused tables.

changelog.d/6047.misc

@@ -0,0 +1,2 @@
+Stop populating some unused tables.
+

synapse/storage/schema/delta/56/drop_unused_event_tables.sql

@@ -1,20 +0,0 @@
-/* Copyright 2019 The Matrix.org Foundation C.I.C.
- *
- * 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.
- */
-
--- these tables are never used.
-DROP TABLE IF EXISTS room_names;
-DROP TABLE IF EXISTS topics;
-DROP TABLE IF EXISTS history_visibility;
-DROP TABLE IF EXISTS guest_access;