Work around issue with CaloFuture MLP initialisaton with debug clang builds

For reasons I do not fully understand, clang(8) in debug mode seems not properly initialise the 'clean' flag for the various Calo MLPs, causing them to fail at runtime with messages like

Problem in class "ReadMLPH": cannot return classifier response because status is dirty

This MR addresses this, by removing the anyway unused virtual interfaces for these classes.