l2tp: remove BUG_ON in l2tp_session_queue_purge

l2tp_session_queue_purge is only called from l2tp_core.c, and it's easy
to statically analyse the code paths calling it to validate that it
should never be passed a NULL session pointer.

Having a BUG_ON checking the session pointer triggers a checkpatch
warning.  Since the BUG_ON is of no value, remove it to avoid the
warning.
Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_core.c

@@ -777,7 +777,6 @@ static int l2tp_session_queue_purge(struct l2tp_session *session)
 {
 	struct sk_buff *skb = NULL;
 
-	BUG_ON(!session);
 	BUG_ON(session->magic != L2TP_SESSION_MAGIC);
 	while ((skb = skb_dequeue(&session->reorder_q))) {
 		atomic_long_inc(&session->stats.rx_errors);