Remove unnecessary mocks from unit test

These mocks where added in the series [1] when we rejected server
operations with qos port. But then we forgot to remove them when
the support is added to those operations during [2].

[1] https://review.opendev.org/q/topic:bp/bandwidth-resource-provider
[2] https://review.opendev.org/q/topic:bp/support-move-ops-with-qos-ports-ussuri

Change-Id: Idbc2d6b2f8f50924dc84683f9701aa28007616af

nova/tests/unit/api/openstack/compute/test_migrate_server.py

@@ -13,7 +13,6 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-import fixtures
 import mock
 from oslo_utils.fixture import uuidsentinel as uuids
 from oslo_utils import uuidutils
@@ -45,9 +44,6 @@ class MigrateServerTestsV21(admin_only_action_common.CommonTests):
         self.stub_out('nova.api.openstack.compute.migrate_server.'
                       'MigrateServerController',
                       lambda *a, **kw: self.controller)
-        self.mock_list_port = self.useFixture(
-            fixtures.MockPatch('nova.network.neutron.API.list_ports')).mock
-        self.mock_list_port.return_value = {'ports': []}
 
     def _get_migration_body(self, **kwargs):
         return {'os-migrateLive': self._get_params(**kwargs)}