Chats: Add cascading delete on both referenced users.

Also remove the now-superfluous join in the chat controller,
which was only used to filter out these cases.

lib/pleroma/web/pleroma_api/controllers/chat_controller.ex

@@ -149,9 +149,7 @@ defmodule Pleroma.Web.PleromaAPI.ChatController do
       from(c in Chat,
         where: c.user_id == ^user_id,
         where: c.recipient not in ^blocked_ap_ids,
-        order_by: [desc: c.updated_at],
-        inner_join: u in User,
-        on: u.ap_id == c.recipient
+        order_by: [desc: c.updated_at]
       )
       |> Repo.all()