Remove expected_success check in images_client
Remove expected_success check in images_client

Because validate_response also check the status code, it is no
need do twice, so remove it for the code clean and consistence.

Change-Id: I1ba34b6b3e43ba416abc696aca043f882b36cea4

openstack/tempest,vedujoshi/tempest,Tesora/tesora-tempest,openstack/tempest,sebrandon1/tempest,Juniper/tempest,Tesora/tesora-tempest,cisco-openstack/tempest,masayukig/tempest,masayukig/tempest,sebrandon1/tempest,cisco-openstack/tempest,Juniper/tempest,vedujoshi/tempest