batman-adv: Remove unused dentry without DEBUGFS

The debug_dir variable in the main structures is only accessed when
CONFIG_BATMAN_ADV_DEBUGFS is enabled. It can be dropped to potentially save
some bytes of memory.

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