Hide the delete form if there are protected objects.

The delete confirmation page musn't show the delete form if there are protected objects.

Here is similar code from [grappelli](https://github.com/sehmaschine/django-grappelli/blob/master/grappelli/templates/admin/delete_confirmation.html#L25-L44) and from [django](https://github.com/django/django/blob/master/django/contrib/admin/templates/admin/delete_confirmation.html#L16-L33).