Little cleanup: let huggingface_hub manage token retrieval
# What does this PR do?

Since [`huggingface_hub==0.11.0` release](https://github.com/huggingface/huggingface_hub/releases/tag/v0.11.0), `hfh` always send the stored token when making requests to the hub, unless explicitly told no (`use_auth_token=False`). Before that, `transformers` had implemented some workarounds to retrieve the cached token when `token=None` is provided by the user. This PR aims to remove those workarounds since `hfh` automatically does this part.

Note: in case of the `PushToHubMixin` class, I changed some the arguments/return value of a private methods (no need to return a token anymore). I thought it would be ok as the method is private but please let me know if you prefer that I revert this part.

## Who can review?

@sgugger

_The documentation is not available anymore as the PR was closed or merged._<|||||>Yes that would be cool if a Great Depreciation is initiated! :) I can help if needed.

It seems the tests are failing with some `ModuleNotFoundError: No module named 'transformers_modules.local'` errors. Is it related to the PR? Doesn't seem to be but since it's affecting tests named `test_from_pretrained_***`, I'm asking.
In any case, if the PR looks good to you, can I let you merge it?<|||||>Those are flaky tests I need to fix :-) Merging!