chore: Remove obsolete files

src/Uno.UI/Mock/DispatcherTimer.net.cs

-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
-
-namespace Windows.UI.Xaml
-{
-	partial class DispatcherTimer
-	{
-		private void StartNative(TimeSpan interval)
-		{
-			; // Not supported in unit tests
-		}
-
-		private void StartNative(TimeSpan dueTime, TimeSpan interval)
-		{
-			; // Not supported in unit tests
-		}
-
-		private void StopNative()
-		{
-			; // Not supported in unit tests
-		}
-	}
-}

src/Uno.UI/Mock/DispatcherTimer.netstdref.cs

-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
-
-namespace Windows.UI.Xaml
-{
-	partial class DispatcherTimer
-	{
-		private void StartNative(TimeSpan interval)
-		{
-			throw new NotSupportedException();
-		}
-
-		private void StartNative(TimeSpan dueTime, TimeSpan interval)
-		{
-			throw new NotSupportedException();
-		}
-
-		private void StopNative()
-		{
-			throw new NotSupportedException();
-		}
-	}
-}

src/Uno.UWP/System/DispatcherQueueTimer.netstd.cs

@@ -26,7 +26,7 @@ partial class DispatcherQueueTimer
 	private void DispatchRaiseTick()
 	{
 #if __WASM__
-		if (!CoreDispatcher.Main.IsThreadingSupported)
+		if (!CoreDispatcher.IsThreadingSupported)
 		{
 			RaiseTick();
 			return;