Removes unused code from wsgi _to_xml_node

Removes support for metadata list_collections and dict_collections
as it is no longer used. Looks like it was last used in the previous
implementation of the Nova API framework (eg before the current V2 one)

Change-Id: I314b13816cdcf51f04926ca9eec67cac765cc893
Merge "Removes unused code from wsgi _to_xml_node"