Remove outdated comments from XSD file

[mbtaylor]

The VOTable.xsd file contained several parts that looked like XML content but were actually commented out, mostly being suggestions for future or relics of the past.
This was especially confusing since the full XSD appears in an Appendix of the VOTable document without syntax highlighting, so you could easily look at something and think it's part of the VOTable schema when it's actually part of an XML comment.

This commit removes such commented-out XML, as well as associated commentary text and some comments (mostly XML comments, but also one xs:documentation element) that constitute musings of the authors of essentially historical interest only.

[mbtaylor]

I have been fairly conservative here - there is a bit more cruft that could be pulled out. The remaining comments are a bit ad hoc and I have not attempted to homogenise them.

In particular the update summary right at the start is not usual for IVOA XSD documents, and provides a fairly patchy picture of the history of the document, which VO archaeologists could presumably examine better using the version control system. So it might be a good idea to remove that as well, but it's fairly harmless and doesn't obscure the rest of the content.

So more action in this area is up for discussion.

[msdemlei]

That's a clear improvement. Thanks!