removes redundant code from echo_server

examples/echo_server/echo_server.cpp

@@ -11,7 +11,7 @@ using websocketpp::lib::placeholders::_2;
 using websocketpp::lib::bind;
 
 // pull out the type of messages sent by our config
-typedef websocketpp::config::asio::message_type::ptr message_ptr;
+typedef server::message_ptr message_ptr;
 
 // Define a callback to handle incoming messages
 void on_message(server* s, websocketpp::connection_hdl hdl, message_ptr msg) {