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 77ddaff2 ("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>