Remove unnecessary methods of wxCocoaOutlineDataSource.

{append,remove}Child() were never used, remove them to make it easier to
remove the use of "children" array itself later (see #16740).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78294 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/osx/cocoa/dataview.h

@@ -314,11 +314,9 @@ private:
     -(void)             removeFromBuffer:(wxPointerObject*)item;
 
     // buffered children handling
-    -(void)             appendChild:(wxPointerObject*)item;
     -(void)             clearChildren;
     -(wxPointerObject*) getChild:(NSUInteger)index;
     -(NSUInteger)       getChildCount;
-    -(void)             removeChild:(NSUInteger)index;
 
     // buffer handling
     -(void) clearBuffers;