fix: CA1823 Unused field

```bash
Warning CA1823 Unused field '_transform' Avalonia.Base (net6.0) .\src\Avalonia.Base\Media\DrawingGroup.cs 110 Active
Warning CA1823 Unused field '_matrix' Avalonia.Base (netstandard2.0) .\src\Avalonia.Base\Media\DrawingContext.cs 280 Active
```

src/Avalonia.Base/Media/DrawingContext.cs

@@ -277,7 +277,6 @@ namespace Avalonia.Media
         private readonly record struct RestoreState : IDisposable
         {
             private readonly DrawingContext _context;
-            private readonly Matrix _matrix;
             private readonly PushedStateType _type;
 
             public enum PushedStateType

src/Avalonia.Base/Media/DrawingGroup.cs

@@ -107,8 +107,6 @@ namespace Avalonia.Media
             private readonly DrawingGroup _drawingGroup;
             private readonly IPlatformRenderInterface _platformRenderInterface = AvaloniaLocator.Current.GetRequiredService<IPlatformRenderInterface>();
 
-            private Matrix _transform;
-
             private bool _disposed;
 
             // Root drawing created by this DrawingContext.