When a remote account is deleted in the fediverse, it sends out a Delete event (I believe). We should handle that event and delete the remote user from our DB whenever that happens.
It might also be helpful to add a shared inbox (TODO: create task), because the server gets pounded when there are multiple followees on the server that the now-deleted account was following.
Delete the actor from remoteusers, and that remoteuser's ID from remotefollows and remoteuserkeys.
An actor Delete activity from Mastodon looks like this (partially):
... "id":"https://mastodon.social/users/UserName#delete", "type":"Delete", "actor":"https://mastodon.social/users/UserName", "to":["https://www.w3.org/ns/activitystreams#Public"], "object":"https://mastodon.social/users/UserName" ...