Replace "netif_default = NULL;" by "netif_set_default(NULL);" in netif_remove (do some processing, like for SNMP...)

src/core/netif.c

@@ -202,7 +202,7 @@ void netif_remove(struct netif * netif)
   /* this netif is default? */
   if (netif_default == netif)
     /* reset default netif */
-    netif_default = NULL;
+    netif_set_default(NULL);
   LWIP_DEBUGF( NETIF_DEBUG, ("netif_remove: removed netif\n") );
 }