[NET_SCHED] sch_cbq: deactivating when grafting, purging etc.

- deactivating of active classes when q.qlen drops to zero
  (cbq_drop)

- a redundant instruction removed from cbq_deactivate_class

PS: probably htb_deactivate in htb_delete and
cbq_deactivate_class in cbq_delete are also
redundant now.

Signed-off-by: Jarek Poplawski <jarkao2@o2.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>