[PKT_SCHED]: Cleanup fifo qdisc and remove unnecessary code

Removes the skb trimming code which is not needed since we never
touch the skb upon failure. Removes unnecessary includes,
initializers, and simplifies the code a bit.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>