net/sched: Retire CBQ qdisc

While this amazing qdisc has served us well over the years it has not been
getting any tender love and care and has bitrotted over time.
It has become mostly a shooting target for syzkaller lately.
For this reason, we are retiring it. Goodbye CBQ - we loved you.

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