Audit samples and docs for references to old APIs (like G+).

[nexushoratio]

Was looking through stuff and discovered that there are still references to Google+:

https://github.com/googleapis/google-api-python-client/tree/main/samples/plus

google-api-python-client/docs/pagination.md

Line 7 in 8b10903

In the following code snippet, the paginated results of a Google Plus activities `list()` method are processed:

google-api-python-client/samples/appengine/main.py

Line 73 in 8b10903

service = discovery.build("plus", "v1", http=http)

See references to removed APIs immediately makes me question the correctness of everything in this repository.