Remove --cert-type option from command line

Remove --cert-type option from command line

With the impending v4.00 release and the soname change, this is a good
time to obsolete the --cert-type option. We've been automatically
detecting key types for a *long* time.

Only remove it from the command line for now; the library never exposed
it, but if the GnuTLS cert-loading code ends up being contributed back
to GnuTLS then they might want something similar there. So leave it in
place but unused.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>