Remove useless attributes m_globalVol, set/getGlobalSound from Game

These do basically nothing. The only time they're used is
getGlobalSound() in an if-statement in main.cpp, but all that
if-conditional does is call setGlobalSound() anyway, which is something
that doesn't really have any side effects. So I'm removing these vars to
simplify the code.

desktop_version/src/Game.cpp

@@ -122,7 +122,6 @@ void Game::init(void)
     musicmuted = false;
     musicmutebutton = 0;
     globalsound = 128;
-    m_globalVol = 1.0f;
 
     glitchrunkludge = false;
     hascontrol = true;