Remove parent/child tests in auto model tests
# What does this PR do?

This PR removes the tests that checks whether models/configs/tokenizers are sorted in the auto mappings such that children classes are before parent classes. While it may have been necessary in the pasts for the `AutoXxx` APIs to work, this is not longer the case and this actually makes it harder on contributors to add new models.

Also, in the future we could have a script like isort that enforces all those auto-mappings are alphabetically sorted.
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_16653). All of your documentation changes will be reflected on that endpoint.