mt76: remove obsolete .add_buf() from struct mt76_queue_ops

It hasn't been used in a while
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt76.h

@@ -152,10 +152,6 @@ struct mt76_queue_ops {
 		     int idx, int n_desc, int bufsize,
 		     u32 ring_base);
 
-	int (*add_buf)(struct mt76_dev *dev, struct mt76_queue *q,
-		       struct mt76_queue_buf *buf, int nbufs, u32 info,
-		       struct sk_buff *skb, void *txwi);
-
 	int (*tx_queue_skb)(struct mt76_dev *dev, enum mt76_txq_id qid,
 			    struct sk_buff *skb, struct mt76_wcid *wcid,
 			    struct ieee80211_sta *sta);