DolphinQt: Remove unneeded Win32 code

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

Source/Core/DolphinQt/Main.cpp

@@ -247,7 +247,6 @@ int main(int argc, char* argv[])
     DolphinAnalytics::Instance().ReportDolphinStart("qt");
 
     Settings::Instance().InitDefaultPalette();
-    Settings::Instance().UpdateSystemDark();
     Settings::Instance().ApplyStyle();
 
     MainWindow win{std::move(boot), static_cast<const char*>(options.get("movie"))};