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.

src/propgrid/props.cpp

@@ -1434,11 +1434,6 @@ bool wxEnumProperty::ValueFromInt_(wxVariant& value, int* pIndex, int intVal, in
     return false;
 }
 
-void
-wxEnumProperty::OnValidationFailure( wxVariant& WXUNUSED(pendingValue) )
-{
-}
-
 void wxEnumProperty::SetIndex( int index )
 {
     m_index = index;