Removes experimental features and fixes non existing include

src/client_main.cpp

 #include "client.hpp"
 #include "definitions.hpp"
-#include "util.h"
 #include "util.hpp"
 #include "wordle-de.pb.h"
 #include <cstdio>

wordle-de.proto

@@ -9,7 +9,7 @@ message Login {
 
 message LoginAck {
   bool loggedIn = 1;
-  optional string error = 2;
+  string error = 2;
 }
 
 message Guess {