CLN: remove unused pandas-gbq tests
Closes #43214

These tests are not run in CI. I'll replace these with tests in the pandas-gbq repo (https://github.com/googleapis/python-bigquery-pandas/issues/379)

- [x] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry (N/A)