Remove migration that refers to a nonexistent column
Remove migration that refers to a nonexistent column

Migrating from scratch hits a snag at this migration, which tries to
drop a column which is referred to nowhere else. I think this is fine
(perhaps I should have just used rake db:schema:load) but may as well
clear this out.

diffux/diffux,kalw/diffux,kalw/diffux,kalw/diffux,diffux/diffux,diffux/diffux