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
diff --git a/tempest/lib/services/compute/images_client.py b/tempest/lib/services/compute/images_client.py
index da8a61e..3dc3749 100644
--- a/tempest/lib/services/compute/images_client.py
+++ b/tempest/lib/services/compute/images_client.py
@@ -61,7 +61,6 @@
     def show_image(self, image_id):
         """Return the details of a single image."""
         resp, body = self.get("images/%s" % image_id)
-        self.expected_success(200, resp.status)
         body = json.loads(body)
         self.validate_response(schema.get_image, resp, body)
         return rest_client.ResponseBody(resp, body)