Cleanup setup.cfg

python-subunit is not used directly anywhere
and it is dependency of both testrepository
and os-testr
(probably was used by some tox wrapper script before)

Change-Id: I04a13a2fd60a57834383bf7b175458176ca7f2e8

setup.cfg

@@ -31,7 +31,6 @@ test =
     hacking>=0.12.0,!=0.13.0,<0.14  # Apache-2.0
     mock>=2.0 # BSD
     python-openstackclient>=3.3.0 # Apache-2.0
-    python-subunit>=0.0.18 # Apache-2.0/BSD
     oslotest>=1.10.0 # Apache-2.0
     reno>=1.8.0 # Apache-2.0
     requests-mock>=1.1 # Apache-2.0