Re #8463. Remove unused DataPointMarker class.
Re #8463. Remove unused PlotEnrichement class.

It was a base to the ArrowMarker & ImageMarker classes, but was not doing
anything. The only place it was specifically mentioned, it was calling
a virtual method on an instance of one of those base classes that is
overridden in those classes.
Re #8463. Remove now-redundant cppcheck suppression statements.