Remove xml parsing codes on api middleware

[hongbin]

Currently, Magnum has already dropped the XML support [1], so we need to remove XML parsing codes in this file https://github.com/openstack/magnum/blob/master/magnum/api/middleware/parsable_error.py .

[1] https://review.openstack.org/#/c/200961/

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/211274

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/211274
Committed: https://git.openstack.org/cgit/openstack/magnum/commit/?id=8a22ddd449dc2f2e3cd6b6a3fc335fe4981023f3
Submitter: Jenkins
Branch: master

commit 8a22ddd449dc2f2e3cd6b6a3fc335fe4981023f3
Author: Akash Gangil <email address hidden>
Date: Mon Aug 10 01:54:23 2015 -0700

    Remove XML parsing code from magnum.

    Openstack no longer supports XML interface for API requests. This
    patch removes any stale code for handling xml api.

    Change-Id: I7f6cdfdde95d757dfab91fc0c18bb606986e5058
    Closes-bug: 1474494