remove dead code: try/import/except for async rest dependencies #15548
Description
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.