How to remove custom attributes which don't appear in the pr

How can I remove custom attributes that don't appear in the process
configuration custom attributes section? Some are not in use at all,
some are duplicates and triplicates that were not properly removed when
customization was done. I suspect we'll need to do some kind of manual
merge for the case where the duplicate or triplicate holds a value, but
the dataset is still small, so this shouldn't be too difficult. I'm
using export to see what custom attributes exist on various work items.

It's curious I can see all of the attributes, including duplicates and
triplicates, when building a query, but don't see these listed in the
Custom Attributes list on the process configuration page.

Thanks, Brian