Retire "ucinewgame" UCI option

UCI protocol it is not clear about what the engine
should be supposed to do when "ucinewgame" is
received. Stockfish simply sets the position to
start FEN, but it is redundant becuase the GUI always
resends the position after "ucinewgame" command, so
it seems we can safely ignore that command.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/uci.cpp

@@ -83,9 +83,6 @@ void uci_loop() {
       else if (token == "go")
           go(pos, is);
 
-      else if (token == "ucinewgame")
-          pos.from_fen(StartFEN, false);
-
       else if (token == "isready")
           cout << "readyok" << endl;