Do not create Object Signing certificate
    
    The Object Signing certificate created during server installation
    was used only for signing the (recently removed) Firefox extension,
    so there's no need to create that certificate any more.
    
    Fixes: https://fedorahosted.org/freeipa/ticket/6399
    Reviewed-By: Martin Basti <mbasti@redhat.com>
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>