Removed clearsigning verification

Removed clearsigning verification

Clearsigning is difficult to verify for all encodings and is not a part
of RFC 3156. Inclusion in this program would encourage its use and lead
users into issues down the road when interacting with other people using
clients that do not support this out of spec feature.

Enigmail uses PGP/MIME by default so users should have an easy time not
creating clearsigned text in their messages unless they decide to paste
clearsigned text straight into a message.

Because the email self-defence guide guides the user through the steps
of installing and setting up enigmail, I am not concerned about leaving
out this feature.