Remove duplicate python-subunit from test-requirements.txt

python-subunit was in both requirements and test-requirements
which pip doesn't like.

Change-Id: I27ccd4cac4e708586d3279722710c867195c373c
diff --git a/test-requirements.txt b/test-requirements.txt
index 2d15daa..53efa46 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -4,7 +4,6 @@
 hacking<0.12,>=0.11.0 # Apache-2.0
 # needed for doc build
 sphinx!=1.3b1,<1.4,>=1.2.1 # BSD
-python-subunit>=0.0.18 # Apache-2.0/BSD
 oslosphinx>=4.7.0 # Apache-2.0
 reno>=1.8.0 # Apache2
 mock>=2.0 # BSD