pppos_client: Fix power_down dce immediately after got IP

This was accidently commited bug when convert to use esp-netif, fix it.

Fixes: 25913af2cc12 ("pppos_client: udated example code to use esp-netif in PPP configuration")
Signed-off-by: Axel Lin <axel.lin@gmail.com>

Merges https://github.com/espressif/esp-idf/pull/4618

examples/protocols/pppos_client/main/pppos_client_main.c

@@ -263,7 +263,6 @@ void app_main(void)
     esp_netif_attach(esp_netif, modem_netif_adapter);
     /* Wait for IP address */
     xEventGroupWaitBits(event_group, CONNECT_BIT, pdTRUE, pdTRUE, portMAX_DELAY);
-    ESP_ERROR_CHECK(dce->power_down(dce));
     /* Config MQTT */
     esp_mqtt_client_config_t mqtt_config = {
         .uri = BROKER_URL,