Remove unused definition in network base.py

The definition os = clients.Manager(interface=cls._interface) has been
replaced by os = cls.get_client_manager().

Change-Id: Idaa33c8f90de901362d6b5a01719dd0be2e700b4
diff --git a/tempest/api/network/base.py b/tempest/api/network/base.py
index dd888a6..5e145e9 100644
--- a/tempest/api/network/base.py
+++ b/tempest/api/network/base.py
@@ -61,7 +61,6 @@
         # Create no network resources for these test.
         cls.set_network_resources()
         super(BaseNetworkTest, cls).setUpClass()
-        os = clients.Manager(interface=cls._interface)
         if not CONF.service_available.neutron:
             raise cls.skipException("Neutron support is required")