[tei-council] Further purification

[Syd Bauman]

The same (probably true) observation makes me think we should make an
effort to use it consistently. :-)

I won't put up a big hissy fit about keeping (after all, we *can* get
the same expressive power from Schematron), but I think org=choice a
powerful ODD construct we should keep. Furthermore, if we allowed
@usage on <attList>, we would have the capability to express "one of
these attributes is required", which seems useful.

> prompts me to wonder whether we should remove that structure from
> ODD anyway, and push the problem back to Schematron. we don’t use
> it consistently.