Remove restrictions on deletion of PublicBodyCategories and PublicBodyHeadings

Allows any `PublicBodyHeading` or `PublicBodyCategory` to be deleted via the admin interface, regardless of whether they are acting as containers for `PublicBodyCategory` or `PublicBody` objects (respectively).

Closes #1969
Nope :(

```
Failure/Error: FactoryGirl.create(:public_body_category, :authority_ids => [authority.id])
NoMethodError:
  undefined method `authority_ids=' for #<PublicBodyCategory:0x7f0e353cdb70>
```

Its not a big deal though.