chore(rel): remove appliance charts (#601)

Remove appliance Helm charts and release process

### Checklist

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

### Test plan
CI 
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Author: jdpleiness