WiFi: partially fix flush() behaviour

We still need to check if trasmission is completed, BTW this commit
should alleviate bugs involving data loss.

See #4029

src/WiFiClient.cpp

@@ -140,8 +140,7 @@ int WiFiClient::peek() {
 }
 
 void WiFiClient::flush() {
-  while (available())
-    read();
+  // TODO: a real check to ensure transmission has been completed
 }
 
 void WiFiClient::stop() {

src/WiFiUdp.cpp

@@ -155,8 +155,7 @@ int WiFiUDP::peek()
 
 void WiFiUDP::flush()
 {
-  while (available())
-    read();
+  // TODO: a real check to ensure transmission has been completed
 }
 
 IPAddress  WiFiUDP::remoteIP()