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
Apparently not:

```
     Failure/Error: pbc = FactoryGirl.create(:public_body_category, :authorities => [authority])
     NoMethodError:
       undefined method `authorities=' for #<PublicBodyCategory:0x7fbc1ed48b68>
```