remove duplicate unit test

looks like this test snuck in during the refactor:

$ diff openstackclient/tests/volume/v1/test_service.py \
  openstackclient/tests/unit/volume/v1/test_service.py

17c17
< from openstackclient.tests.volume.v1 import fakes as service_fakes
---
> from openstackclient.tests.unit.volume.v1 import fakes as service_fakes

Change-Id: I769b2d39f28cfaf65e4027c785b0ddbbefa26aea

Author: stevemar