Unnecessary GKE cluster credentials setup

[ex-nerd]

A lot of the GKE cluster credentials stuff for ~/.kube/config in Tutorials/Kubernetes/GKE.md should be able to be dealt with automatically via the command that Cloud Shell, which now(?) automatically installs all of the necessary credentials/endpoint/etc into the kube config:

gcloud container clusters get-credentials $CLUSTER_NAME --zone $ZONE --project $PROJECT_NAME

This also supercedes the username/password configuration in the doc (which no longer exists) and drastically simplifies setup.