sched: replace __skb_dequeue with __qdisc_dequeue_head

After previous patch these functions are identical.
Replace __skb_dequeue in qdiscs with __qdisc_dequeue_head.

Next patch will then make __qdisc_dequeue_head handle
single-linked list instead of strcut sk_buff_head argument.

Doesn't change generated code.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>