lib/modules: Finally remove deprecated types.optionSet

[infinisil]

Description of changes

types.optionSet has been deprecated for almost 10 years now (0e33368)! A removal was already attempted in 2019 (#54637), but it was promptly reinstantiated since some third-party uses were discovered (#56857).

It's finally time to remove it for good :)

Ping @roberth @danbst @rycee

Things done

[roberth]

pkgs/test/mkOption still tries to reference option sets, but it is horribly bitrotten; essentially untouched since '08.
It is safe to remove, but begs the question whether option merging is sufficiently tested in lib/tests/modules.sh? The test case description comments don't seem to reference such requirements.

[infinisil]

@roberth Damn, I didn't know about that ancient test case. I don't see any reliance on optionSet, but I removed that in another commit, it's not useful anymore.

Yeah option merging might be undertested, though we can be reasonably sure it won't break due to the NixOS tests making use of a lot of NixOS modules with various merging operations. Wouldn't hurt to have more specific test cases though, but out of scope for this PR.