Should aux factories have a coord-system ?

[pp-mo]

Summary:
Review whether the coord_system of a factory could be removed.

Background:
While working to enhance documentation of #3583 changes,
noticed that the coord_system property of a factory is a separate implementation from that on a Coord.

In fact, I suspect the coord_system for all AuxFactory-s is always None, as they are only used to represent vertical coords, and Iris coord-systems are only used for horizontal.
So this could be removed ??

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