DolphinQt: Remove unneeded Win32 code

Dolphin already uses QStyleHints::colorScheme to detect dark mode,
so this code is now redundant.

Source/Core/DolphinQt/Settings.h

@@ -53,8 +53,6 @@ public:
   // UI
   void TriggerThemeChanged();
   void InitDefaultPalette();
-  void UpdateSystemDark();
-  void SetSystemDark(bool dark);
   bool IsSystemDark();
   bool IsThemeDark();