net-sched: sch_htb: remove child and sibling lists

Now that the qdisc isn't destroyed in hierarchical order anymore,
the only user of the child lists left is htb_parent_last_child().
This can be easily changed to use a counter of children to save
a few bytes.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>