Remove broken migrations

It seems these should never have been migrations in the first place.
It's considered an anti-pattern to include model code in your migrations
(these referred to User and Update), and we can see why -- having
renamed the models, these migrations now fail.

Apparently the Right Thing is to write a rake task to do the work in
question, and then run it via whatever deployment mechanism you use.
Since these have already been run on our dev server, and they're not all
that important anyway (i.e. won't break people's local machines they're
developing on, etc), and on top of all that they're a couple of months
old so most active developers should have run them already... as I was
saying, considering all that, I'm just blowing them away.