Revert "[16.0] fastapi: optionally authenticated partner"

[lmignon]

Reverts #387

[sbidoul]

@lmignon do you want to merge this according to #387 (comment)

[lmignon]

@sbidoul I still have to think about it. The problem is not so trivial. Even if you declare the partner_id parameter as optional using type annotations, the implementation of the chosen method won't be aware of this annotation, which means that it will fail in the case of auth_jwt, for example, because it won't have found a token. Despite everything, I'm not comfortable with the proposed approach because I feel like I'm missing something. If I don't find another solution, the current code makes sense. If not, this PR could be merged with an explanation of how to cover this use case.