fix(modem): Fix netif data race causing PPP startup delays

Removes PPP_started signal on reception, since lwip handles data
reception if the netif is up/down (which we correctly set in start()
stop() methods)

Closes https://github.com/espressif/esp-protocols/issues/308

components/esp_modem/include/cxx_include/esp_modem_netif.hpp

@@ -54,9 +54,10 @@ public:
      */
     void stop();
 
-private:
     void receive(uint8_t *data, size_t len);
 
+private:
+
     static esp_err_t esp_modem_dte_transmit(void *h, void *buffer, size_t len);
 
     static esp_err_t esp_modem_post_attach(esp_netif_t *esp_netif, void *args);