remove unused pip requirements (bug 1027340)

https://bugzilla.mozilla.org/show_bug.cgi?id=1027340

r? @andymckay @robhudson
we never had any of these requirements in our `prod.txt`:

* distribute
* kombu
* stevedore
* wsgiref

I thought we needed to include all of the dependencies of our dependencies? so how did celery work then?

(I still think it's crazy we have to put our dependencies' dependencies in this list.)