CLN: remove `private_key` and `verbose` from gbq
Since `verbose` and `private_key` have been deprecated since https://github.com/pandas-dev/pandas/pull/30200, we should also remove both from function since they are not used at all? see them in `read_gbq` and `to_gbq`, e.g.

https://github.com/pandas-dev/pandas/blob/24857a2844be9ee6f604d93ea18301f587751f0a/pandas/io/gbq.py#L33

https://github.com/pandas-dev/pandas/blob/24857a2844be9ee6f604d93ea18301f587751f0a/pandas/io/gbq.py#L34