batman-adv: remove redundant hard_iface assignment

The batadv_neigh_node_new() function already sets the hard_iface pointer.

Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Acked-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>