20470: Fix discovery document generation to drop unpublished fields

20470: Fix discovery document generation to drop unpublished fields

Now uses the list of API published fields (selectable_attributes) to
generate discovery doc, this causes some obsolete and nonpublic fields
to disappear from the discovery doc (but actually they were never part
of the public API in the first place).

The immediate reason to do this is because workbench 1 was using the
discovery document to craft a list of fields to select, but the
changes to the way select work in this branch means that asking for
unpublished fields now throws an error.

Arvados-DCO-1.1-Signed-off-by: Peter Amstutz <peter.amstutz@curii.com>