Revert "[Doctests] Refactor doctests + add CI"
Reverts huggingface/transformers#22987

This PR created a hard dependency on `pytest` which we don't want in Transformers. Looking a bit more it would be better if the whole `doctest_utils.py` module lived outside of the Transformers library, so it should be structured differently.
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_23245). All of your documentation changes will be reflected on that endpoint.