Remove Istio pkill command for Istio <1.3

[linjmeyer]

Istio 1.3 introduced the /quitquitquit endpoint for triggering an Istio shutdown. Prior to that a pkill command had to be used. Istio 1.3 is now end of life, I think we can remove the pkill shutdown feature as all supported version of Istio have the better option of using the quit endpoint.