net-sched: sch_htb: remove write-only qdisc filter_cnt

The filter_cnt is supposed to count filter references to a class.
Since the qdisc can't be the target of a filter, it doesn't need
a filter_cnt. In fact the counter is never decreased since cls_api
considers a return value of zero a failure and doesn't unbind again.

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