Remove remaining dependency on apitools

apitools is deprecated and continued reliance on it is complicating Beam's dependency management. Previous work should have already removed our dependence on the clients it generates for us (see #25676, #25678, #25679, #25680 and (tracking issue:) #19073).

Delete these clients (if not already deleted) and remove the remaining references to apitools outside of the clients by either replacing them with calls to [Google Cloud Python](https://github.com/googleapis/google-cloud-python) libraries or (for small things), by bringing them into the Beam codebase if acceptable external replacements can't be found.

Finally, remove Beam's dependency on apitools. The job is done when this can be done safely.

### Issue Priority

Priority: 2 (default / most feature requests should be filed as P2)

### Issue Components

- [X] Component: Python SDK
- [ ] Component: Java SDK
- [ ] Component: Go SDK
- [ ] Component: Typescript SDK
- [ ] Component: IO connector
- [ ] Component: Beam examples
- [ ] Component: Beam playground
- [ ] Component: Beam katas
- [ ] Component: Website
- [ ] Component: Spark Runner
- [ ] Component: Flink Runner
- [ ] Component: Samza Runner
- [ ] Component: Twister2 Runner
- [ ] Component: Hazelcast Jet Runner
- [ ] Component: Google Cloud Dataflow Runner