Get rid of unnecessary overriding function

OnValidationFailure() in derived class wxEnumProperty has the same implementation (empty body) as the implementation in the base class wxPGProperty so overriding this function in derived class is not necessary.

interface/wx/propgrid/props.h

@@ -361,8 +361,6 @@ public:
     // this take advantage of it.
     virtual int GetChoiceSelection() const;
 
-    virtual void OnValidationFailure( wxVariant& pendingValue );
-
 protected:
 
     int GetIndex() const;