DB/WOEID cleanup

[dgw]

Back in the Ancient Days, this plugin used to store users' locations in a key named woeid. That's not used any more, and it might make sense to have a cleanup routine to delete those unused rows from Sopel's database.

Debatable whether it should be done automatically (e.g. in setup()) or on command by a Sopel admin. I suppose there could be other plugins that also use the same key name, so a Super Ambitious Epic Automatic Solution™ would have to determine somehow that no other installed plugin uses the same key name, and abort the cleanup if one does. (Good luck with that? 🤣)