Remove unused code in test_attach_interfaces.py

"body = jsonutils.dumps({'port_id': FAKE_PORT_ID1})" is unused in
test_attach_interface_without_network_id(). We should remove it.

Change-Id: Id31b559752d57c9f55fd2bf0de909b8d8f9a0333
Closes-Bug: #1266617
Merge "Remove unused code in test_attach_interfaces.py"