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
Also wonder whether you can use the [expect change matcher](https://www.relishapp.com/rspec/rspec-expectations/v/2-14/docs/built-in-matchers/expect-change). I don't know whether you'd have do do this though:

```ruby
it "destroys a public body heading that has associated categories" do
  expect {
    category = FactoryGirl.create(:public_body_category)
    # …
    post :destroy, { :id => @heading.id }
    # …
  }.to change(PublicBodyHeading.count).by(1)
end
```

…which is a bit ugly.