ofproto: Do not update stats on fake bond interface.

There are couple of reasons to remove this support:
*   This is used in very old OVS use-case. It is much better
    to read stats directly from OVS.
*   Forthcoming commit will remove support for setting stats
    for vport. The stats update depends on stats-set.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>