batman-adv: Drop unused function batadv_hardif_remove_interfaces()

The function batadv_hardif_remove_interfaces was meant to remove all
interfaces which are currently in the list of known (compatible) hardifs
during module unload. But the function unregister_netdevice_notifier is
called in batadv_exit before batadv_hardif_remove_interfaces. This will
trigger NETDEV_UNREGISTER events for all available interfaces and in this
process remove all interfaces from batadv_hardif_list. And
batadv_hardif_remove_interfaces only operated on this (empty) list.

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