Remove wsgiref from requirements.txt

wsgiref is part of python standard library since 2.5
so having it in requirements.txt has no impact.
https://docs.python.org/2/whatsnew/2.5.html#the-wsgiref-package

Change-Id: I42da2984ed4b0b0f0537274423413e1e7ff2c061

requirements.txt

@@ -9,7 +9,6 @@ eventlet>=0.16.1
 PasteDeploy>=1.5.0
 Routes>=1.12.3,!=2.0
 WebOb>=1.2.3
-wsgiref>=0.1.2
 kombu>=2.5.0
 lockfile>=0.8
 psutil>=1.1.1,<2.0.0