Remove prepare_instance_network in ImagesOneServerNegativeTestJSON

In ImagesOneServerNegativeTestJSON, all testcases are negative and
have no relationship with "ssh into the server", so there is no
need to use prepare_instance_network in setup_credentials. This is
to remove it for code clean.

Change-Id: I7de581e19ff9eb02473dd2f26bb833e2d270fd06
diff --git a/tempest/api/compute/images/test_images_oneserver_negative.py b/tempest/api/compute/images/test_images_oneserver_negative.py
index d9b80e1..039283a 100644
--- a/tempest/api/compute/images/test_images_oneserver_negative.py
+++ b/tempest/api/compute/images/test_images_oneserver_negative.py
@@ -69,11 +69,6 @@
             raise cls.skipException(skip_msg)
 
     @classmethod
-    def setup_credentials(cls):
-        cls.prepare_instance_network()
-        super(ImagesOneServerNegativeTestJSON, cls).setup_credentials()
-
-    @classmethod
     def setup_clients(cls):
         super(ImagesOneServerNegativeTestJSON, cls).setup_clients()
         cls.client = cls.compute_images_client