net/sched: Remove uapi support for ATM qdisc

Commit fb38306c ("net/sched: Retire ATM qdisc") retired the ATM qdisc.
Remove UAPI for it. Iproute2 will sync by equally removing it from user space.

Reviewed-by: Victor Nogueira <victor@mojatatu.com>
Reviewed-by: Pedro Tammela <pctammela@mojatatu.com>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>