Remove unnessary delete_volume in test_volumes_list

In test_volumes_list.py, it calles cls.create_volume to
create 3 test volumes, and volumes created by cls.create_volume
will be automatically cleaned by base's clear_volumes() in
base's resource_cleanup, so this is to remove the unnecessary
delete_volume

Change-Id: I763cb6cc66cebab7601846c2009676c20a8d1fdd
1 file changed