Pushing to an open PR should (hopefully) cancel preexisting actions.
Remove dead CI requirements.

Currently, creating async clients with constructor works fine:

from google.cloud import vision

client = vision.ImageAnnotatorAsyncClient()
print(type(client))
prints: <class 'google.cloud.vision_v1.services.image_annotator.async_client.ImageAnnotatorAsyncClient'>

Using from_service_account_* methods on the other hand return sync client:

client = vision.ImageAnnotatorAsyncClient.from_service_account_file("service_account.json")
print(type(client))
This gives <class 'google.cloud.vision_v1.services.image_annotator.client.ImageAnnotatorClient'>, which is just a normal synchronous client.

This happens because these functions are linked to sync clients' methods. Now it's fixed to call the same method, but with proper class.

Otherwise the client may throw an exception if the server is newer than the client (i.e. some of the returned messages have new fields).
A tracking bug: #780

🤖 I have created a release \*beep\* \*boop\* 
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### [0.40.10](https://www.github.com/googleapis/gapic-generator-python/compare/v0.40.9...v0.40.10) (2021-02-17)


### Bug Fixes

* ignore unknown fields returned from server for REST ([#777](https://www.github.com/googleapis/gapic-generator-python/issues/777)) ([a70b078](https://www.github.com/googleapis/gapic-generator-python/commit/a70b0787f7e3d40642a4f68574f0cc493cc4e054))
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