rbd: remove unnecessary asserts in rbd_dev_image_probe()

spec->image_id assert doesn't buy us much and image_format is asserted
in rbd_dev_header_name() and rbd_dev_header_info() anyway.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>