[Task]: Remove archaic PubSubSource and PubSubSink

### What needs to happen?

Reading from and writing to PubSub should be proper (if primitive) transforms, rather than being represented by "special" parameters of Read/Write operations necessitating workarounds like https://github.com/apache/beam/blob/release-2.48.0/sdks/python/apache_beam/io/iobase.py#L946.

### Issue Priority

Priority: 3 (nice-to-have improvement)

### Issue Components

- [X] Component: Python SDK
- [ ] Component: Java SDK
- [ ] Component: Go SDK
- [ ] Component: Typescript SDK
- [X] Component: IO connector
- [ ] Component: Beam examples
- [ ] Component: Beam playground
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