Remove hpa-example from sample manifests

[`gke-scheduled-autoscaler/k8s/deployment-example.yaml`](https://github.com/GoogleCloudPlatform/kubernetes-engine-samples/blob/main/gke-scheduled-autoscaler/k8s/deployment-example.yaml#L35) 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 https://github.com/kubernetes/release/issues/2924#issuecomment-1454301010, 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