Axe NETWORK_[set/get]AchievementProgress()

These are unused.

Ethan originally added them in case Terry wanted achievement
percentages. But he didn't add them, and I don't think the achievements
are changing anytime soon, so it's safe to remove this dead code.

desktop_version/src/GOGNetwork.c

@@ -23,13 +23,4 @@ void GOG_unlockAchievement(const char *name)
 {
 }
 
-int32_t GOG_getAchievementProgress(const char *name)
-{
-    return 0;
-}
-
-void GOG_setAchievementProgress(const char *name, int32_t stat)
-{
-}
-
 #endif /* MAKEANDPLAY */