remove unused kill-garden-server option in fake_network_plugin
remove unused kill-garden-server option in fake_network_plugin

[#168370717]

Co-authored-by: Giuseppe Capizzi <1968455d84ef56ceb93e8ec69a3034a6fe0ab9cc@pivotal.io>

cloudfoundry-incubator/guardian,cloudfoundry-incubator/guardian,cloudfoundry-incubator/guardian