v1.4.1: Remember to drop unused tables

[richvdh]

#5893 dropped four tables (room_names, topics, history_visibility, guest_access) from the database schema, but #6047 reinstated them. We should remember to re-redelete them.

Note that there will exist servers which got upgraded to develop and will have already run the delta file, and hence dropped the tables, so any future delta which re-drops them should bear this in mind. This will be WAY EASIER if we can do it before the database schema version bumps up to 57 so that we can just reinstate the old delta.

[richvdh]

done in #6115