Remove functionality to create OpenPGP certificate for OpenPGP card keys

It's a niche feature (because usually the OpenPGP certificate is generated
together with the OpenPGP card keys) and creating a second OpenPGP
certificate for OpenPGP card keys will result in two certificates with
identical fingerprints which is a sure way to cause confusion.

(cherry picked from commit bd5fb6e84bbab429d88e967792017e3d65bb01f0)

• GnuPG-bug-id: T7113