remove dead code: `try/import/except` for async rest dependencies

[vchudnov-g]

async_rest has been an optional dependency., leading to installation of google-auth[aiohttp] >= 2.35.0. WBut now we unconditionally required google_-auth >= 2.35.0. We should determine whether we can get rid of the extra, and whether we can get rid of the tr/import/except pattern in multiple places that tests for the async REST dependencies.

This is motivated by a PR conversation.

[daniel-sanche]

I think keeping aiohttp as an optional dependency makes sense, because google-auth's AsyncAuthorizedSession will raise an exception on use if it's not provided

But the try/except blocks should be removed if we pin to v2.35, because they will never hit that ImportError

(The missing aiohttp error is raised when AsyncAuthorizedSession is called, not at ImportTime)