batman-adv: remove unused inline function batadv_arp_change_timeout

There's no callers in-tree.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>