Remove unused client in SecurityGroupRulesNegativeTestJSON

There is still an unused client in SecurityGroupRulesNegativeTestJSON,
this is to remove it.

Change-Id: If365c92f2a8d4a53b03ff2fdf5c95b4a4d58ddb5
diff --git a/tempest/api/compute/security_groups/test_security_group_rules_negative.py b/tempest/api/compute/security_groups/test_security_group_rules_negative.py
index 4f53663..32b3ea3 100644
--- a/tempest/api/compute/security_groups/test_security_group_rules_negative.py
+++ b/tempest/api/compute/security_groups/test_security_group_rules_negative.py
@@ -35,7 +35,6 @@
     @classmethod
     def setup_clients(cls):
         super(SecurityGroupRulesNegativeTestJSON, cls).setup_clients()
-        cls.client = cls.security_groups_client
         cls.rules_client = cls.security_group_rules_client
 
     @test.attr(type=['negative'])