pkt_sched: sch_qfq: remove unused member of struct qfq_sched

The member (u32) "num_active_agg" of struct qfq_sched has been unused
since its introduction in 462dbc91
"pkt_sched: QFQ Plus: fair-queueing service at DRR cost" and (AFAICT)
there is no active plan to use it; this removes the member.

Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
Acked-by: Paolo Valente <paolo.valente@unimore.it>
Signed-off-by: David S. Miller <davem@davemloft.net>