Remove `google-generativeai` as a dependency.

The `google-generativeai` package is deprecated, and emits warnings when
it's imported. I'm doing some cleanup work to remove it from popular
surfacess, so here's a PR :)

I'll note that `litellm` uses direct HTTP requests for the API, so it
does not use `google-genai` (except for type checking) or
`google-generativeai` (except for a deprecated PaLM codepath).