fq_codel: remove set but not used variables 'prev_ecn_mark' and 'prev_drop_count'

Fixes gcc '-Wunused-but-set-variable' warning:

net/sched/sch_fq_codel.c: In function fq_codel_dequeue:
net/sched/sch_fq_codel.c:288:23: warning: variable prev_ecn_mark set but not used [-Wunused-but-set-variable]
net/sched/sch_fq_codel.c:288:6: warning: variable prev_drop_count set but not used [-Wunused-but-set-variable]

They are not used since commit 77ddaff218 ("fq_codel: Kill
useless per-flow dropped statistic")

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_fq_codel.c

@@ -285,7 +285,6 @@ static struct sk_buff *fq_codel_dequeue(struct Qdisc *sch)
 	struct sk_buff *skb;
 	struct fq_codel_flow *flow;
 	struct list_head *head;
-	u32 prev_drop_count, prev_ecn_mark;
 
 begin:
 	head = &q->new_flows;
@@ -302,9 +301,6 @@ begin:
 		goto begin;
 	}
 
-	prev_drop_count = q->cstats.drop_count;
-	prev_ecn_mark = q->cstats.ecn_mark;
-
 	skb = codel_dequeue(sch, &sch->qstats.backlog, &q->cparams,
 			    &flow->cvars, &q->cstats, qdisc_pkt_len,
 			    codel_get_enqueue_time, drop_func, dequeue_func);