[media] staging: lirc: remove redundant NULL check in unregister_from_lirc()

"ir" is already checked before calling unregister_from_lirc().

Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>