net_sched: cbq: remove a flaky use of qdisc_is_throttled()

So far no qdisc ever unset the throttled bit at enqueue() time,
so CBQ usage of qdisc_is_throttled() was flaky.

Since __QDISC_STATE_THROTTLED set/unset is way too expensive
considering that only CBQ was eventually caring for this status,
it would make sense to implement a Qdisc ops ->is_throttled()
if we find that this is needed.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>