Remove hpa-example from sample manifests

[shannonxtreme]

gke-scheduled-autoscaler/k8s/deployment-example.yaml uses k8s.gcr.io/hpa-example as the image. It's not used in any of our docs, so I think this should be ok to change.

In kubernetes/release#2924 (comment), it's suggested that hpa-example is going to be removed in the near future. We should replace the image in this repo with either a forked version or a different image.

cc @bourgeoisor

[bourgeoisor]

@rbarberop FYI
In https://cloud.google.com/kubernetes-engine/docs/how-to/multidimensional-pod-autoscaling#creating_a_deployment, hpa-example was changed for hello-app.

[bourgeoisor]

@rbarberop I was looking into usage of that file (https://github.com/GoogleCloudPlatform/kubernetes-engine-samples/blob/main/gke-scheduled-autoscaler/k8s/deployment-example.yaml) and don't see it being used in the associated tutorial (https://cloud.google.com/kubernetes-engine/docs/tutorials/reducing-costs-by-scaling-down-gke-off-hours). Is it okay to remove deployment-example.yaml from the repo (see reasoning above)? Or is it used elsewhere?

[rbarberop]

LGTM @bourgeoisor