Optimize reloadData in RoomsTableViewController

I just noticed that we now have the case that we call `self.tableView reloadData` twice when `refreshRoomList` is called and we don't have a search string. Once it's done inside of `filterRooms` and once in `refreshRoomList`. Not sure if this will be a problem in the real world or if iOS might even optimize the two calls, but we should keep that in mind and optimize that in the long run.

_Originally posted by @SystemKeeper in https://github.com/nextcloud/talk-ios/pull/1289#pullrequestreview-1513322065_