batman-adv: Drop deprecated debugfs support

The debugfs support in batman-adv was marked as deprecated by the commit
00caf6a2 ("batman-adv: Mark debugfs functionality as deprecated") and
scheduled for removal in 2021.

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