remove broken ./tag support

remove broken ./tag support

The commit that added the (working) support for using /tag to override
tagbase also tried to make ./tag work. Problem is, tags are links,
and ./foo is not a valid link (though I think there's a wishlist about it).

So, using ./tag really resulted in tag creation links that led to a
"bad page name" error. And even if the tag were created in the right place,
the link didn't go to it.