Remove discover from test-requirements

It's only needed for python < 2.7 which is not supported

Change-Id: Ie70dd2c7e7f1eb9859bc1e080704211844ca134d

test-requirements.txt

@@ -5,7 +5,6 @@
 hacking<0.11,>=0.10.0
 
 coverage>=3.6
-discover
 python-subunit>=0.0.18
 sphinx!=1.2.0,!=1.3b1,<1.3,>=1.1.2
 oslosphinx>=2.5.0 # Apache-2.0