Evaluate is cloudevent dependency is still needed

[steren]

This sample depends on github.com/cloudevents/sdk-go/v2 and has a lot of logic to parse CloudEvent.

This is source of potential vulnerability and code complexity.

It's unclear that this sample still needs to be able to handle CloudEvent.

We should figure out if it is used in any EventArc sample or if it used to demo EventArc.

If not, we should remove this dependency and code.

[steren]

I see it is still used here: https://cloud.google.com/eventarc/docs/run/create-trigger-storage-console#event-receiver