batman-adv: Drop implicit creation of batadv net_devices

The sysfs code in batman-adv was could create a new batadv interfaces on
demand when a string (interface name) was written to the
batman-adv/mesh_iface file. But the code no longer exists in the current
batman-adv codebase. The helper code to implement this behavior must be
considered as unused and can be dropped.

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