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.h

@@ -436,8 +436,6 @@ public:
     void returntoeditor(void);
 #endif
 
-    int gametimer;
-
     bool inline inspecial(void)
     {
         return inintermission || insecretlab || intimetrial || nodeathmode;