emacs: Drop content-free "Unknown signature status" button

When we have not been able to evaluate the signature status of a given
MIME part, showing a content-free (and interaction-free) "[ Unknown
signature status ]" button doesn't really help the user at all, and
takes up valuable screen real-estate.

A visual reminder that a given message is *not* signed isn't helpful
unless it is always present, in which case we'd want to see "[ Unknown
signature status ]" buttons on all messages, even ones that don't have
a signing structure, but i don't think we want that.

Amended by db to drop the unused initialization of 'label'