Rename Menu::setslowdown2 to Menu::setslowdown

Now that setslowdown1 has been removed it's no longer necessary to have
a 2 on the end of setslowdown2.

desktop_version/src/Game.cpp

@@ -6753,7 +6753,7 @@ void Game::createmenu( enum Menu::MenuName t )
         menuxoff = -30;
         menuyoff = 64;
         break;
-    case Menu::setslowdown2:
+    case Menu::setslowdown:
         option("normal speed");
         option("80% speed");
         option("60% speed");