Documentation: remove reference to pktcdvd in cdrom documentation

pktcdvd got killed in a previous commit, remove the reference to it as
well in the cdrom documentation.

Fixes: 1cea5180 ("block: remove pktcdvd driver")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Jens Axboe <axboe@kernel.dk>