Remove wxListCtrl::EnableSortIndicator()

The old API seems unnecessarily complex, it is simpler to just let the
application call ShowSortIndicator() itself from its
wxEVT_LIST_COL_CLICK handler, which needs to be defined anyhow in order
to actually sort the items, rather than require it to enable sort
indicator, explicitly set it initially and then remember to not set it
any longer in response to the column clicks.

Also make RemoveSortIndicator() non-virtual and implement it simply as
ShowSortIndicator(-1) because this actually simplifies the code too.

include/wx/generic/private/listctrl.h

@@ -381,7 +381,6 @@ public:
     int m_colToSend;
     int m_widthToSend;
 
-    bool m_enableSortCol;
     bool m_sortAsc;
     int m_sortCol;