drop mock from test-requirements
drop mock from test-requirements

after commit I8bbcedb7ad3f0bc2e06dfa13878a97411ee1dc6d switched usage
to unittest.mock everywhere we can now drop mock from test-requirements
which reduces extra package from being installed

Change-Id: Id9d87aa8449b46dd74b2acedcdd9239200d108fb

openstack/keystone,openstack/keystone,openstack/keystone,mahak/keystone,mahak/keystone,mahak/keystone