Remove TEMPEST_ENABLE_NOVA_XML_API from Tempest config

I5b3e221d942e09134024b82acaf179dc869357e0 has removed xml_api_v2 from
Tempest config, and xml_api_v2 setting of DevStack is unnecessary now.
This patch removes it.

Change-Id: Ic85fbfda13fd352c4d513859915aaca27e894b58
Merge "Remove TEMPEST_ENABLE_NOVA_XML_API from Tempest config"