Get rid of wxListCtrl::m_count in wxMSW.

This simplifies the code and fixes the bug with GetItemCount() returning wrong
(old) value in wxEVT_LIST_INSERT_ITEM event handler as m_count wasn't updated
by then yet.

Closes #3793.

include/wx/msw/listctrl.h

@@ -409,8 +409,6 @@ protected:
 
     int               m_colCount;   // Windows doesn't have GetColumnCount so must
                                     // keep track of inserted/deleted columns
-    long              m_count;      // Keep track of item count to save calls to
-                                    // ListView_GetItemCount
 
     // all wxMSWListItemData objects we use
     wxVector<wxMSWListItemData *> m_internalData;