[StoreKit] Remove unsupported SKAdNetwork code from .NET. (#13268)

src/StoreKit/SKAdNetworkCompat.cs

@@ -14,14 +14,10 @@ using System.Runtime.Versioning;
 using Foundation;
 using ObjCRuntime;
 
-#if TVOS && !XAMCORE_4_0
+#if TVOS && !NET
 namespace StoreKit {
 	[Obsolete ("Not usable from tvOS and will be removed in the future.")]
-#if NET
-	[UnsupportedOSPlatform ("tvos")]
-#else
 	[Unavailable (PlatformName.TvOS)]
-#endif
 	public class SKAdNetwork : NSObject {
 
 		[BindingImpl (BindingImplOptions.GeneratedCode | BindingImplOptions.Optimizable)]
@@ -41,4 +37,4 @@ namespace StoreKit {
 		public static void RegisterAppForAdNetworkAttribution () => throw new NotSupportedException ();
 	}
 }
-#endif
+#endif // TVOS && !NET