[appkit] Remove leftover code made obsolete by newrefcount.

Remove code that we needed at some point due to memory related issues. These
issues were fixed several years ago (newrefcount), so we no longer need this
code (in fact it causes problems, see xamarin#15089).

src/appkit.cs

@@ -15929,16 +15929,12 @@ partial interface NSView : NSDraggingDestination, NSAnimatablePropertyContainer,
 		void WillRemoveSubview ([NullAllowed] NSView subview);
 
 		[Export ("removeFromSuperview")]
-		[PreSnippet ("var mySuper = Superview;", Optimizable = true)]
-		[PostSnippet ("if (mySuper != null) {\n\t#pragma warning disable 168\n\tvar flush = mySuper.Subviews;\n#pragma warning restore 168\n\t}", Optimizable = true)]
 		void RemoveFromSuperview ();
 
 		[Export ("replaceSubview:with:")][PostGet ("Subviews")]
 		void ReplaceSubviewWith (NSView oldView, NSView newView);
 
 		[Export ("removeFromSuperviewWithoutNeedingDisplay")]
-		[PreSnippet ("var mySuper = Superview;", Optimizable = true)]
-		[PostSnippet ("if (mySuper != null) {\n\t#pragma warning disable 168\n\tvar flush = mySuper.Subviews;\n#pragma warning restore 168\n\t}", Optimizable = true)]
 		void RemoveFromSuperviewWithoutNeedingDisplay ();
 
 		[Export ("resizeSubviewsWithOldSize:")]