Delete tries to delete this entry from its feed. If the entry
does not contain a link rel="edit", we throw an error (either
the entry does not yet exist or this is not an editable
feed). If we have a link rel="edit", we do the empty-body
HTTP DELETE to that URI and check for a response of 2xx.
Usually the response would be 204 No Content, but the Atom
Publishing Protocol permits it to be 200 OK.