Inline duplicate service fixture with test

Similarly to the previous commit, this fixture is only used once,
so can benefit from being inline with its test.

tests/conftest.py

@@ -587,25 +587,6 @@ def mock_create_service(mocker):
         'app.service_api_client.create_service', side_effect=_create)
 
 
-@pytest.fixture(scope='function')
-def mock_create_duplicate_service(mocker):
-    def _create(
-        service_name,
-        organisation_type,
-        message_limit,
-        restricted,
-        user_id,
-        email_from,
-    ):
-        json_mock = Mock(return_value={'message': {'name': ["Duplicate service name '{}'".format(service_name)]}})
-        resp_mock = Mock(status_code=400, json=json_mock)
-        http_error = HTTPError(response=resp_mock, message="Default message")
-        raise http_error
-
-    return mocker.patch(
-        'app.service_api_client.create_service', side_effect=_create)
-
-
 @pytest.fixture(scope='function')
 def mock_update_service(mocker):
     def _update(service_id, **kwargs):