Query optimization in taxonomy_node_form

The current query on which taxonomy_node_form is based is of the form:

SELECT v.*, n.type FROM [...]

But the code does not make use of any of the results except v.vid, using DB server resources uselessly. Patch reduces the query to

SELECT v.vid FROM [...]

This reduces the size of the result set and, to some extent, the work to be performed by the DB server.