batman-adv: Drop documentation about debugfs files

The debugfs files were marked as deprecated by commit 00caf6a2
("batman-adv: Mark debugfs functionality as deprecated"). 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>