Don't leak associations in wxCocoaDataViewControl

Fix ClearColumns() to release associations between m_OutlineView and
native Cocoa control when re-creating the latter. Previously it only
created a new association, but left the old one with no-deleted native
control in the map.

Fixes #17485.

include/wx/osx/core/private.h

@@ -343,6 +343,7 @@ public :
                         FindBestFromWXWidget(WXWidget control);
     
     static void         RemoveAssociations( wxWidgetImpl* impl);
+    static void         RemoveAssociation(WXWidget control);
 
     static void         Associate( WXWidget control, wxWidgetImpl *impl );