Handle service_del with bad service name
    
    The command 'ipa service-del badservice' used to fail with an internal
    server error, because check_required_principal() could not handle a
    principal that is not a service principal. All del commands have less
    strict error checking of primary keys so they can reference any stored
    key, even illegal ones.
    
    check_required_principal() skips required principal check if the
    principal is not a service principal. A non-service principal can never
    be a required principal.
    
    Fixes: https://pagure.io/freeipa/issue/7793
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>