Drop support for --gce-service-account, require activated creds

hack/ginkgo-e2e.sh

@@ -112,7 +112,6 @@ export PATH=$(dirname "${e2e_test}"):"${PATH}"
   --provider="${KUBERNETES_PROVIDER}" \
   --gce-project="${PROJECT:-}" \
   --gce-zone="${ZONE:-}" \
-  --gce-service-account="${GCE_SERVICE_ACCOUNT:-}" \
   --gke-cluster="${CLUSTER_NAME:-}" \
   --kube-master="${KUBE_MASTER:-}" \
   --cluster-tag="${CLUSTER_ID:-}" \