remove null sms sender test

it's no longer possible for an sms_sender to be null

tests/app/service/test_rest.py

@@ -1925,22 +1925,6 @@ def test_update_service_does_not_call_send_notification_when_restricted_not_chan
     assert not send_notification_mock.called
 
 
-def test_update_service_works_when_sms_sender_is_null(sample_service, client, mocker):
-    sample_service.sms_sender = None
-    data = {'name': 'new name'}
-
-    resp = client.post(
-        'service/{}'.format(sample_service.id),
-        data=json.dumps(data),
-        headers=[create_authorization_header()],
-        content_type='application/json'
-    )
-
-    assert resp.status_code == 200
-    # make sure it wasn't changed to not-null under the hood
-    assert sample_service.sms_sender is None
-
-
 def test_search_for_notification_by_to_field_filters_by_status(client, notify_db, notify_db_session):
     create_notification = partial(
         create_sample_notification,