api-ref: remove notion of supported xml format
api-ref: remove notion of supported xml format

Neutron API does not support XML format for awhile. JSON is the only
supported format right now.

Those remnants came from Ic0c2348d7db941deaec6e1d110954fe5893c2265 that
was merged into api-ref recently. The contents were somewhat abandoned
before, so cruft like that piled up.

Change-Id: I50c90c13f390e16d14aaf3c310b8356b56d4d2f4

openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib