aio: remove the needless registration of ring file's private_data

Remove the registration of ring file's private_data, we do not use
it.

Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>