Remove wxListBox::MacGetBlockEvents()

Just test m_blockEvents directly, there doesn't seem to be any gain in
using an accessor here.

Also test it only once instead of doing it twice in
MacHandleSelectionChange().

include/wx/osx/listbox.h

@@ -122,8 +122,6 @@ public:
 
     wxListWidgetImpl* GetListPeer() const;
 
-    bool MacGetBlockEvents() const { return m_blockEvents; }
-
     virtual void HandleLineEvent( unsigned int n, bool doubleClick );
 
     // This is called by wxNSTableView