lib/modules: Finally remove deprecated types.optionSet

types.optionSet has been deprecated for almost 10 years now
(0e333688ce)! A removal
was already attempted in 2019
(27982b408e), but it was promptly
reinstantiated since some third-party uses were discovered
(f531ce75e4178c6867cc1d0f7fec96b2d5c3f1cb).

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

lib/options.nix

@@ -79,8 +79,6 @@ rec {
     visible ? null,
     # Whether the option can be set only once
     readOnly ? null,
-    # Deprecated, used by types.optionSet.
-    options ? null
     } @ attrs:
     attrs // { _type = "option"; };