delete the hardcoded license list in `datasets`

[julien-c]

Feel free to delete the license list in datasets [...]

Also FYI in #4926 I also removed all the validation steps anyway (language, license, types etc.)

Originally posted by @lhoestq in #4930 (comment)

[...], in my opinion we can just delete this file from datasets, the validation is happening hub-side anyways now?

Originally posted by @julien-c in #4930 (comment)