[XKit] Adopt XAMCORE_4_0 changes in .NET. (#14027)

This just removes obsolete API.

src/XKit/XKitCompat.cs

@@ -1,4 +1,4 @@
-#if !XAMCORE_4_0
+#if !NET
 using System;
 using Foundation;
 using ObjCRuntime;
@@ -40,4 +40,4 @@ namespace UIKit {
 #endif // WATCH
 
 }
-#endif
+#endif // NET