mDNS: remove netif from mDNS structs

it's more practical to use netif as a wrapper instead of wrapping
netif in the mdns packets. netif contains all information.
Netif is passed along were needed.

src/include/lwip/apps/mdns_out.h

@@ -73,7 +73,7 @@ extern "C" {
 /* Lookup for text info on service instance */
 #define REPLY_SERVICE_TXT       0x80
 
-err_t mdns_send_outpacket(struct mdns_outmsg *msg);
+err_t mdns_send_outpacket(struct mdns_outmsg *msg, struct netif *netif);
 void mdns_prepare_txtdata(struct mdns_service *service);
 
 #endif /* LWIP_MDNS_RESPONDER */

src/include/lwip/apps/mdns_priv.h

@@ -125,8 +125,6 @@ struct mdns_outpacket {
 
 /** mDNS output message */
 struct mdns_outmsg {
-  /** Netif to send the packet on */
-  struct netif *netif;
   /** Identifier. Used in legacy queries */
   u16_t tx_id;
   /** dns flags */