[CLEANUP] Remove security_level option from Authentications

The option security_level option was deprecated
since 4.7 and can now be removed.

Also do some cleanup in related code.

Releases: 6.2
Resolves: #56256
Change-Id: I48dcb788ca654aea14fb7125128c564fd373b550
Reviewed-on: https://review.typo3.org/27825
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel