The ChangeVisitIdTypeOfAhoyEventsToInteger from 364be5541 is not very well done.
You can not change the schema without changing possibly existing data.
Ahoy events used some kind of string as `visit_id`. Changing this column
to integrer will just barf up `Mysql2::Error: Truncated incorrect INTEGER value`.
So we need a way for people who are in this situation now to get out of it.
Luckily we completely dropped ahoy events a while ago so we can just delete them.
