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.

src/generic/listctrl.cpp

@@ -5464,21 +5464,6 @@ wxColour wxGenericListCtrl::GetForegroundColour() const
     return m_mainWin ? m_mainWin->GetForegroundColour() : wxColour();
 }
 
-bool wxGenericListCtrl::DoPopupMenu( wxMenu *menu, int x, int y )
-{
-#if wxUSE_MENUS
-    // Coordinates here are given in this control coordinates system, but
-    // PopupMenu() wants them in the coordinates system of the window it's
-    // called on, so transform them.
-    ClientToScreen(&x, &y);
-    m_mainWin->ScreenToClient(&x, &y);
-
-    return m_mainWin->PopupMenu( menu, x, y );
-#else
-    return false;
-#endif
-}
-
 wxSize wxGenericListCtrl::DoGetBestClientSize() const
 {
     // The base class version can compute the best size in report view only.