net/sched: Remove uapi support for dsmark qdisc

Commit bbe77c14

 ("net/sched: Retire dsmark qdisc") retired the dsmark
classifier. Remove UAPI support 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>