net: sched: sch_tbf: Don't overwrite backlog before dumping

In 2011, in commit b0460e44 ("sch_tbf: report backlog information"),
TBF started copying backlog depth from the child Qdisc before dumping, with
the motivation that the backlog was otherwise not visible in "tc -s qdisc
show".

Later, in 2016, in commit 8d5958f4 ("sch_tbf: update backlog as well"),
TBF got a full-blown backlog tracking. However it kept copying the child's
backlog over before dumping.

That line is now unnecessary, so remove it.

As shown in the following example, backlog is still reported correctly:

    # tc -s qdisc show dev veth0 invisible
    qdisc tbf 1: root refcnt 2 rate 1Mbit burst 128Kb lat 82.8s
     Sent 505475370 bytes 406985 pkt (dropped 0, overlimits 812544 requeues 0)
     backlog 81972b 66p requeues 0
    qdisc bfifo 0: parent 1:1 limit 10Mb
     Sent 505475370 bytes 406985 pkt (dropped 0, overlimits 0 requeues 0)
     backlog 81972b 66p requeues 0

Signed-off-by: Petr Machata <petrm@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>