chore/release: Sunset AMIs (#823)

closes PLAT-21
closes PLAT-25

This PR removes the `.github/workflows/release-amis.yml` workflow which
will stop publication new AMI images. Currently the `sourcegraph/deploy`
repo receives updates as a side effect of the helm release. This will
disable that and Sunset new AMI releases.

### Checklist

- [ ] Follow the [manual testing
process](https://github.com/sourcegraph/deploy-sourcegraph-helm/blob/main/TEST.md)
- [ ] Update
[changelog](https://github.com/sourcegraph/deploy-sourcegraph-helm/blob/main/charts/sourcegraph/CHANGELOG.md)
- [ ] Update [Kubernetes update
doc](https://docs.sourcegraph.com/admin/updates/kubernetes)

### Test plan

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Author: DaedalusG