[Tagging] Remove non-prefixed versions of 'contact:' scheme

[Cj Malone]

> But not all of them are necessarily contacts.  I've added URLs for
> historic buildings that give more information about the
> building.  There is nobody to talk to about it.  I've added websites
> for companies; there is a contact page on that website but the URL
> I've given is for the company website as a whole.

Surely that's an argument for new tags as well as contact:website, for
example description:website where a user agent could give users a "Read
more" link. A website tag is generic, which has the obvious benefit of
used widely and easily, but more precise tags like contact:website give
user agents much more flexibility.