Fix outdated comments in SynthesisToCifConverter regarding explicit kinds

SynthesisToCifConverter contains:

        // Remove requirements.
        //
        // Note that requirement automata have already been relabeled as
        // supervisors, and are thus not removed.
        //
        // Requirement invariants have all been given an explicit kind, so they
        // can be removed here, even though the component in which they are
        // specified may have changed its kind.
        //
        // Whether it is allowed to remove the requirements depends on the BDD
        // predicate simplification option.

There are no implicit/explicit kinds anymore, as we removed the deprecated kind inheritance feature a while back. See #139 (closed). These comments need to be updated.