Don't use unique indexes with polymorphics

With a mix of scoped and unscoped slugs, because the indexes are created on
the top-level class, you'll end up with `duplicate key error`s on the unscoped
index even when scoped slugs would be valid.

I can't think of a way to spec this, since mongoid_slug is otherwise doing the right thing, and the error will only be visible in safe mode.