Obsolete Bacterium, or place it correctly

[cmungall]

bacterium is currently a sibling of cellular organism. If we keep it, we should make it a subclass.

But why do we have it? It seems arbitrary. Why not prokaryote or eukaryote? There are no domain or range constraints on it. If people want to classify an organism instance using a kingdom, order, etc they can simply use type with an NCBITaxon node