Remove Nova v3 XML tests completely

We have decided that XML support is removed from Nova v3 API and
the tests also are removed from Tempest.
Most code has been removed from Tempest, but there are some code
related to Nova v3 XML tests yet.

This patch removes them from Tempest.

Part of bp:remove-v3-xml-api

Change-Id: Ie3c595fb77143e58e30404c42f358d6443a52c0b
Merge "Remove Nova v3 XML tests completely"