remove unused pip requirements (bug 1027340)

https://bugzilla.mozilla.org/show_bug.cgi?id=1027340

r? @andymckay @robhudson
https://github.com/tomchristie/django-rest-framework/blob/b3b0515ae6325e7de6a582bbb2fd7eeac687c325/rest_framework/parsers.py#L148

IARC parses XML. DRF uses defusedxml.