Add cop for use of remove_column

What does this MR do?

Adds a cop for use of remove_column in a non-down step of a regular (not post-deployment) migration. This will cause downtime if we don't catch it! https://docs.gitlab.com/ee/development/what_requires_downtime.html#dropping-columns

Are there points in the code the reviewer needs to double check?

The specs are pretty meh.

Why was this MR needed?

It's way too easy to miss this in review.