net/sched: Retire ATM qdisc

The ATM qdisc has served us well over the years but has not been getting much
TLC due to lack of known users. Most recently it has become a shooting target
for syzkaller. For this reason, we are retiring it.

Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Acked-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>