batman-adv: Drop batadv_priv_debug_log struct

The support for the batman-adv ring buffer for debug logs was dropped with
the removal of the debugfs filesystem. The structure storing this ring
buffer is therefore no longer needed since commit aff6f5a6
("batman-adv: Drop deprecated debugfs support")

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