Update MainWindow.xaml.cs

Author: ghost1372

dev/DevWinUI.Gallery/Views/MainWindow.xaml.cs

@@ -1,5 +1,4 @@
-using DevWinUI.Gallery;
-using Microsoft.UI.Windowing;
+using Microsoft.UI.Windowing;
 using Microsoft.UI.Xaml.Input;
 using WinRT;