batman-adv: remove obsolete variable primary_iface from orig_node

This variable became obsolete when changing to the new bonding mechanism
based on the multi interface optimization. Since its not used anywhere,
remove it.

Reported-by: Linus Lüssing <linus.luessing@web.de>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>