lwip_socket_init() is not needed any more -> compatibility define

src/api/sockets.c

@@ -241,15 +241,6 @@ static void event_callback(struct netconn *conn, enum netconn_evt evt, u16_t len
 static void lwip_getsockopt_internal(void *arg);
 static void lwip_setsockopt_internal(void *arg);
 
-/**
- * Initialize this module. This function has to be called before any other
- * functions in this module!
- */
-void
-lwip_socket_init(void)
-{
-}
-
 /**
  * Map a externally used socket index to the internal socket representation.
  *