cdrom: hide CONFIG_CDROM menu selection

We don't need to expose this. The point is that drivers select
the uniform CDROM layer, if they need it, the user should not
have to make a conscious decision on whether to include this
separately or not.

Fixes: 2a750166

 ("block: Rework drivers/cdrom/Makefile")
Signed-off-by: Jens Axboe <axboe@kernel.dk>