batman-adv: Drop deprecated sysfs support

The sysfs in batman-adv support was marked as deprecated by the commit
42cdd521 ("batman-adv: ABI: Mark sysfs files as deprecated") and
scheduled for removal in 2021.

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