[TASK] Cleanup EXT:openid

* Remove deprecated calls to require_once
* Restore license header
* Remove empty TODO file

Resolves: #56832
Releases: 6.2
Change-Id: I291f5469638659d64e9df181017d1588f27317e1
Reviewed-on: https://review.typo3.org/28309
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind