batman-adv: Remove orig_node reference handling from send_skb_unicast

The function batadv_send_skb_unicast is not acquiring a reference for an
orig_node nor removing it from any datastructure. It still reduces the
reference counter for an object which is still in the hands of the caller.

This is confusing and can lead in the future to problems in the reference
handling of the caller function.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Acked-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>