Environment- ClearPass 6.4.x and earlier
Answer- At present deleting ClearPass Endpoint Attributes via XML API is not possible. This option would be available in the 6.5.1 code of ClearPass. With XML API at present, we could only delete the complete endpoint all together.
From GUI we can add/remove Endpoint attributes.
Alternatively, we can also export the existing endpoints (XML file) and edit the XML file for bulk edit/update.