net_sched: sch_fq: remove one obsolete check in fq_dequeue()

After commit eeb84aa0

 ("net_sched: sch_fq: do not assume EDT
packets are ordered"), all skbs get a non zero time_to_send
in flow_queue_add()

This means @time_next_packet variable in fq_dequeue()
can no longer be zero.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>