[AdSupport] Adopt XAMCORE_4_0 changes in .NET. (#13887)

src/AdSupport/ASCompat.cs

@@ -1,4 +1,4 @@
-#if !XAMCORE_4_0
+#if !NET
 
 using System;
 using System.Runtime.Versioning;
@@ -8,9 +8,6 @@ namespace AdSupport {
 	public  partial class ASIdentifierManager {
 
 #if MONOMAC
-#if NET
-		[UnsupportedOSPlatform ("macos")]
-#endif
 		[Obsolete ("Empty stub. This member was retroactively marked as unavailable for macOS.")]
 		public virtual void ClearAdvertisingIdentifier ()
 		{