Remove unnecessary Cinder v1 code

Since the commit I04de2d90fe11f8ec4acbfa983bc0ad3aa979f45b Cinder v1
API tests have been removed. So there is not any case which requires
_api_version is 1. This patch just removes it.

TrivialFix

Change-Id: I7af71ef6eada3017bc5a96830515915b7a848a74
diff --git a/tempest/api/volume/test_volumes_list.py b/tempest/api/volume/test_volumes_list.py
index a852cea..df98720 100644
--- a/tempest/api/volume/test_volumes_list.py
+++ b/tempest/api/volume/test_volumes_list.py
@@ -78,11 +78,7 @@
                 params=params)['volumes']
 
         # Validating params of fetched volumes
-        # In v2, only list detail view includes items in params.
-        # In v1, list view and list detail view are same. So the
-        # following check should be run when 'with_detail' is True
-        # or v1 tests.
-        if with_detail or self._api_version == 1:
+        if with_detail:
             for volume in fetched_vol_list:
                 for key in params:
                     msg = "Failed to list volumes %s by %s" % \