requirements: Remove explicit python-subunit dependency
requirements: Remove explicit python-subunit dependency

We don't rely on this ourselves and stestr will bring it in for us.

Change-Id: I51f305ac080c41463081e7039421d238b81f5d95
Signed-off-by: Stephen Finucane <492121341a95b3c3aab646bed44634f739dd019b@redhat.com>

openstack/cliff,openstack/cliff