Fix broken test (which relied on default timeout being None).

The upstream behaviour has changed such that None is no longer the
default:

googleapis/google-auth-library-python#435

Author: rsgowman

tests/test_db.py

@@ -739,7 +739,6 @@ def test_valid_db_url(self, url):
         assert ref._client.timeout is None
         assert ref.get() == {}
         assert len(recorder) == 1
-        assert recorder[0]._extra_kwargs.get('timeout') is None
 
     @pytest.mark.parametrize('url', [
         None, '', 'foo', 'http://test.firebaseio.com', 'https://google.com',