batman-adv: Simplify 'batadv_v_ogm_aggr_list_free()'

Use 'skb_queue_purge()' instead of re-implementing it.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>