Support push_to_hub without org/user to default to logged-in user
This behavior is aligned with:
- the behavior of `datasets` before merging #6519
  - the behavior described in the corresponding docstring
- the behavior of `huggingface_hub.create_repo`

Revert "Support push_to_hub canonical datasets (#6519)"
- This reverts commit a887ee78835573f5d80f9e414e8443b4caff3541.

Fix #6597.