net/sched: fq_codel: Avoid set-but-unused variable

The code introduced by commit 2ccccf5f

 ("net_sched: update
hierarchical backlog too") only sets prev_backlog in fq_codel_dequeue()
but not using that anywhere, remove that setting.

Cc: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>