Remove unnecessary check on registration returned.

Right now the ACME client checks that the returned registration matches the
registation posted, but there's no guarantee this will always be the case, and
this only introduces unnecessary fragility.

acme/acme/client.py

@@ -89,8 +89,6 @@ class Client(object):  # pylint: disable=too-many-instance-attributes
         :returns: Registration Resource.
         :rtype: `.RegistrationResource`
 
-        :raises .UnexpectedUpdate:
-
         """
         new_reg = messages.NewRegistration() if new_reg is None else new_reg
         assert isinstance(new_reg, messages.NewRegistration)
@@ -101,12 +99,7 @@ class Client(object):  # pylint: disable=too-many-instance-attributes
 
         # "Instance of 'Field' has no key/contact member" bug:
         # pylint: disable=no-member
-        regr = self._regr_from_response(response)
-        if (regr.body.key != self.key.public_key() or
-                regr.body.contact != new_reg.contact):
-            raise errors.UnexpectedUpdate(regr)
-
-        return regr
+        return self._regr_from_response(response)
 
     def _send_recv_regr(self, regr, body):
         response = self.net.post(regr.uri, body)