This will make django automatically remove them when we run
squashmigrations. There are still some RunSQL statements which
we will have to take care of manually.
This cleans up any messages that might have been exchanged with
`NEW_USER_BOT` or `FEEDBACK_BOT` (cross-realm bots that were last
used, as far as we know, years ago) that have been completely removed
from the codebase.
Details on the algorithm are in the migration code itself.
Fixes#13583.