Remove game.gametimer in favor of game.frames

PR #279 added game.gametimer solely for the editor ghosts feature. It
seems that whoever originally wrote it (Leo for the now-dead VVVVVV:
Community Edition, I believe) forgot that the game already had its own
timer, that they could use.

The game timer does increment on unfocus pause (whereas this doesn't),
but that's a separate issue, and it ought to not do that.

desktop_version/src/Game.cpp

@@ -352,7 +352,6 @@ void Game::init(void)
     skipfakeload = false;
 
     ghostsenabled = false;
-    gametimer = 0;
 
     cliplaytest = false;
     playx = 0;