nbd: Remove nbd_co_flush() wrapper function

The only thing nbd_co_flush() does is call nbd_client_co_flush(). Just
use that function directly in the BlockDriver definitions and remove the
wrapper.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20230504115750.54437-10-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>