sch_htb: do not report fake rate estimators

When I prepared commit d250a5f9 ("pkt_sched: gen_estimator: Dont
report fake rate estimators"), htb still had an implicit rate estimator
for all its classes.

Then later, I made this rate estimator optional in commit 64153ce0
("net_sched: htb: do not setup default rate estimators"), but I forgot
to update htb use of gnet_stats_copy_rate_est()

After this patch, "tc -s qdisc ..." no longer report fake rate
estimators for HTB classes.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>