Stop forwarding DoPopupMenu() to wxGenericListCtrl main window

There doesn't seem to be anything wrong with just showing the popup menu
in this window itself and this avoids the need to do any coordinate
adjustments, which didn't work correctly for the menus shown over the
header window after the previous commit (and hadn't worked correctly for
the menus shown over the non-header area before it).

See #18133.

include/wx/generic/listctrl.h

@@ -211,8 +211,6 @@ protected:
     long DoInsertColumn(long col, const wxListItem& info) wxOVERRIDE;
 
 
-    virtual bool DoPopupMenu( wxMenu *menu, int x, int y ) wxOVERRIDE;
-
     virtual wxSize DoGetBestClientSize() const wxOVERRIDE;
 
     // return the text for the given column of the given item