rbd: don't NULL out ->obj_request in rbd_img_obj_parent_read_full()

If rbd_img_request_submit() fails, parent_request->obj_request is
NULLed out, triggering an assert in rbd_obj_request_put():

  rbd_img_request_put(parent_request)
    rbd_parent_request_destroy
      rbd_obj_request_put(NULL)

Just remove it -- parent_request->obj_request will be put in
rbd_parent_request_destroy().

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>