remove vestigal support for KRL signatures

remove vestigal support for KRL signatures

When the KRL format was originally defined, it included support for
signing of KRL objects. However, the code to sign KRLs and verify KRL
signatues was never completed in OpenSSH.

Now, some years later, we have SSHSIG support in ssh-keygen that is
more general, well tested and actually works. So this removes the
semi-finished KRL signing/verification support from OpenSSH and
refactors the remaining code to realise the benefit - primarily, we
no longer need to perform multiple parsing passes over KRL objects.

ok markus@