sch_netem: Remove classful functionality

Patrick McHardy noticed that: "a lot of the functionality of netem
requires the inner tfifo anyways and rate-limiting is usually done
on top of netem. So I would suggest so either hard-wire the tfifo
qdisc or at least make the assumption that inner qdiscs are
work-conserving.", and later: "- a lot of other qdiscs still don't
work as inner qdiscs of netem [...]".

So, according to his suggestion, this patch removes classful options
of netem. The main reason of this change is to remove ops->requeue()
method, which is currently used only by netem.

Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>