Remove discover from test-requirements
Remove discover from test-requirements

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

Change-Id: I0772abfa0343e755e50938f8384a9a58c46e4412

noironetworks/python-group-based-policy-client,openstack/python-group-based-policy-client,noironetworks/python-group-based-policy-client,stackforge/python-group-based-policy-client,openstack/python-group-based-policy-client,stackforge/python-group-based-policy-client