feat: 🎸 remove /valid endpoint
not used anymore by the Hub, and not meant to be public.

Waiting for:
- [x] https://github.com/huggingface/datasets-server/pull/1784
- [x] https://github.com/huggingface/moon-landing/pull/7456 (internal)
feat: 🎸 remove /valid endpoint: not used anymore by the Hub, and not meant to be public.

Waiting for:
- [x] https://github.com/huggingface/datasets-server/pull/1784
- [x] https://github.com/huggingface/moon-landing/pull/7456 (internal)