remove krb_rdns setting
    
    The krb_rdns setting only applies to Koji's old-style Kerberos
    authentication. It has no effect now because Fedora uses the newer-style
    GSSAPI authentication.
    
    Remove the setting to reduce confusion for new Koji users.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1812702
    
    Signed-off-by: Ken Dreyer <kdreyer@redhat.com>