[Foundation] NSCoding and NS[Mutable]Copying shouldn't be models. (#21257)

It doesn't make sense for NSCoding and NS[Mutable]Copying to be models,
so remove those attributes for XAMCORE_5_0.

src/foundation.cs

@@ -2776,8 +2776,10 @@ namespace Foundation {
 		bool IsPartialStringValid (ref string partialString, out NSRange proposedSelRange, string origString, NSRange origSelRange, [NullAllowed] out string error);
 	}
 
+#if !XAMCORE_5_0
 	[BaseType (typeof (NSObject))]
 	[Model]
+#endif
 	[Protocol]
 	interface NSCoding {
 		// [Abstract]
@@ -2794,8 +2796,10 @@ namespace Foundation {
 		// note: +supportsSecureCoding being static it is not a good "generated" binding candidate
 	}
 
+#if !XAMCORE_5_0
 	[BaseType (typeof (NSObject))]
 	[Model]
+#endif
 	[Protocol]
 	interface NSCopying {
 		[Abstract]
@@ -2804,8 +2808,10 @@ namespace Foundation {
 		NSObject Copy ([NullAllowed] NSZone zone);
 	}
 
+#if !XAMCORE_5_0
 	[BaseType (typeof (NSObject))]
 	[Model]
+#endif
 	[Protocol]
 	interface NSMutableCopying : NSCopying {
 		[Abstract]