batman-adv: Drop documentation about sysfs files

The sysfs files will be marked as deprecated in the near future. They
are already replaced by the batadv generic netlink family. The
documentation should not advertise its usage anymore and instead
promote the generic netlink family and a userspace tool to access it.

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