cdrom: Remove unnecessary NULL check before unregister_sysctl_table()

cdrom: Remove unnecessary NULL check before unregister_sysctl_table()

unregister_sysctl_table() checks for NULL pointers internally.
Remove unneeded NULL check here.

Signed-off-by: Chen Ni <nichen@iscas.ac.cn>
Link: https://lore.kernel.org/lkml/20250514032139.2317578-1-nichen@iscas.ac.cn
Reviewed-by: Phillip Potter <phil@philpotter.co.uk>
Link: https://lore.kernel.org/lkml/aCURuvkmz-fw3Nnp@equinox
Signed-off-by: Phillip Potter <phil@philpotter.co.uk>
Link: https://lore.kernel.org/r/20250514223354.1429-2-phil@philpotter.co.uk
Signed-off-by: Jens Axboe <axboe@kernel.dk>