Remove discover from test-requirements

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

Change-Id: If0e9fd59c47d27bc58917500c554819c6aa30b2c

Author: Swapnil Kulkarni (coolsvap)

test-requirements.txt

@@ -4,7 +4,6 @@
 hacking<0.11,>=0.10.0
 
 coverage>=3.6 # Apache-2.0
-discover # BSD
 fixtures>=3.0.0 # Apache-2.0/BSD
 mock>=2.0 # BSD
 oslosphinx!=3.4.0,>=2.5.0 # Apache-2.0