blockdev: remove unused paio parameter documentation

This field isn't present anymore.
Signed-off-by: NJohn Snow <jsnow@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20190223000614.13894-10-jsnow@redhat.com
Signed-off-by: NJohn Snow <jsnow@redhat.com>

blockdev.c

@@ -1257,7 +1257,6 @@ out_aio_context:
  * @node: The name of the BDS node to search for bitmaps
  * @name: The name of the bitmap to search for
  * @pbs: Output pointer for BDS lookup, if desired. Can be NULL.
- * @paio: Output pointer for aio_context acquisition, if desired. Can be NULL.
  * @errp: Output pointer for error information. Can be NULL.
  *
  * @return: A bitmap object on success, or NULL on failure.