Change disconnect function (#466)

* Change disconnect function

Current implementation does not disconnect the WiFi. Use of `esp_wifi_disconnect()` does.

* Update WiFiSTA.cpp

put back the clear ssid + password
remove the duplicate `esp_wifi_set_config(WIFI_IF_STA, &conf);`

libraries/WiFi/src/WiFiSTA.cpp

@@ -262,7 +262,7 @@ bool WiFiSTAClass::disconnect(bool wifioff)
     WiFi.getMode();
     esp_wifi_start();
     esp_wifi_set_config(WIFI_IF_STA, &conf);
-    ret = esp_wifi_set_config(WIFI_IF_STA, &conf) == ESP_OK;
+    ret = esp_wifi_disconnect() == ESP_OK;
 
     if(wifioff) {
         WiFi.enableSTA(false);