Remove consultation publication type

This cleans up some crufty code that was causing me some WTF. There doesn't appear to be any reason for `PublicationType::Consultation` to exist anymore - for a start, a `Consultation` is not a `Publication`, and doesn't require a type to identify itself as a consultation. I should imagine that we'll want to update /government/consultations at some point so that consultations have their own filtering like announcements and policies, rather than masquerading them as publications...