Remove XML support from admin_password V3 API plugin

Removes all XML support from the V3 API admin_password
plugin.

Partially implements blueprint remove-v3-xml-api

Change-Id: Ib45a04d34704bc259ef6e457d6dfb6f79c929edf
DocImpact: XML no longer suported for the admin_password plugin
Merge "Remove XML support from admin_password V3 API plugin"