batman-adv: remove useless find_router look up

This is not used anymore with the new fragmentation, and it might
actually mess up the bonding code because find_router() assumes it
is only called once per packet.

Signed-off-by: Simon Wunderlich <simon@open-mesh.com>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>