batman-adv: Drop legacy code for auto deleting mesh interfaces

The only way to automatically drop batadv mesh interfaces when all soft
interfaces were removed was dropped with the sysfs support. It is no longer
needed to have them handled by kernel anymore.

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