remove unused pip requirements (bug 1027340)

https://bugzilla.mozilla.org/show_bug.cgi?id=1027340

r? @andymckay @robhudson
@washort we using `oauth2`? in [AMO](https://github.com/mozilla/olympia/blob/master/apps/api/tests/test_oauth.py#L34) we are. I see we using `oauthlib`. and then we're using `oauth2client` in [this GA script you wrote](https://github.com/mozilla/olympia/blob/master/scripts/auth_google_analytics.py). which one is that? this is all a bit confusing.