fix/release: remove appliance image publication (#800)

I'm not sure how this hasn't failed on other releases but we don't
publish appliance anymore and this crashed promotion in the 6.12.0
release:
```

👉 [      step] Running step "sg ops:migrator"
--
[sg ops:migrator] +++ 👉 using Docker Hub registry index.docker.io/sourcegraph
[sg ops:migrator] +++ 👉 pinning images to tag "6.12.0"
[sg ops:migrator] Updated migrator
[sg ops:migrator]   - docker.io/sourcegraph/migrator:6.0.0@sha256:ec295eb0b743da6bf56777ca6524972267a5c442b0288095e2fe12fce38ebacc
[sg ops:migrator]   + index.docker.io/sourcegraph/migrator:6.12.0@sha256:b12184766122a8f22cef78f8ecbfd5cd20837f9d480716cea0e031e9f505b1f1
[      step] Step "sg ops:migrator" succeeded
👉 [      step] Running step "sg ops:appliance"
[sg ops:appliance] +++ 👉 using Docker Hub registry index.docker.io/sourcegraph
[sg ops:appliance] +++ 👉 pinning images to tag "6.12.0"
[sg ops:appliance] ^^^ +++  ⨯ couldn't read charts/sourcegraph-appliance/values.yaml: open charts/sourcegraph-appliance/values.yaml: no such file or directory
[sg ops:appliance] Step failed: exit status 1
🚨 Error: The command exited with status 1

```

https://buildkite.com/sourcegraph/deploy-sourcegraph-helm/builds/2441#019bb42a-f692-41cc-9c2e-d3f27c6c6d95

### 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

<!--
As part of SOC2/GN-104 and SOC2/GN-105 requirements, all pull requests
are REQUIRED to
provide a "test plan". A test plan is a loose explanation of what you
have done or
implemented to test this, as outlined in our Testing principles and
guidelines:

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  Write your test plan here after the "Test plan" header.
-->

Author: DaedalusG

release.yaml

@@ -575,34 +575,20 @@ promoteToPublic:
             --docker-password $DOCKER_PASSWORD \
             charts/sourcegraph-migrator/
 
-      - name: sg ops:appliance
-        cmd: |
-          set -eu
-          registry=index.docker.io/sourcegraph
-          sg ops update-images \
-            -t {{inputs.server.tag}} \
-            -k helm \
-            --registry "${registry}" \
-            --docker-username $DOCKER_USERNAME \
-            --docker-password $DOCKER_PASSWORD \
-            charts/sourcegraph-appliance/
-
       - name: "chart:version"
         cmd: |
           comby -matcher ".generic" -in-place "version: \":[~\d+\.\d+\.\d+]\"" 'version: "{{inputs.server.tag}}"' -f charts/sourcegraph/Chart.yaml
           comby -matcher ".generic" -in-place "version: \":[~\d+\.\d+\.\d+]\"" 'version: "{{inputs.server.tag}}"' -f charts/sourcegraph/examples/subchart/Chart.yaml
           comby -matcher ".generic" -in-place "version: \":[~\d+\.\d+\.\d+]\"" 'version: "{{inputs.server.tag}}"' -f charts/sourcegraph-executor/dind/Chart.yaml
           comby -matcher ".generic" -in-place "version: \":[~\d+\.\d+\.\d+]\"" 'version: "{{inputs.server.tag}}"' -f charts/sourcegraph-executor/k8s/Chart.yaml
           comby -matcher ".generic" -in-place "version: \":[~\d+\.\d+\.\d+]\"" 'version: "{{inputs.server.tag}}"' -f charts/sourcegraph-migrator/Chart.yaml
-          comby -matcher ".generic" -in-place "version: \":[~\d+\.\d+\.\d+]\"" 'version: "{{inputs.server.tag}}"' -f charts/sourcegraph-appliance/Chart.yaml
 
       - name: "chart:appVersion"
         cmd: |
           comby -matcher ".generic" -in-place "appVersion: \":[~\d+\.\d+\.\d+]\"" 'appVersion: "{{inputs.server.tag}}"' -f charts/sourcegraph/Chart.yaml
           comby -matcher ".generic" -in-place "appVersion: \":[~\d+\.\d+\.\d+]\"" 'appVersion: "{{inputs.server.tag}}"' -f charts/sourcegraph-executor/dind/Chart.yaml
           comby -matcher ".generic" -in-place "appVersion: \":[~\d+\.\d+\.\d+]\"" 'appVersion: "{{inputs.server.tag}}"' -f charts/sourcegraph-executor/k8s/Chart.yaml
           comby -matcher ".generic" -in-place "appVersion: \":[~\d+\.\d+\.\d+]\"" 'appVersion: "{{inputs.server.tag}}"' -f charts/sourcegraph-migrator/Chart.yaml
-          comby -matcher ".generic" -in-place "appVersion: \":[~\d+\.\d+\.\d+]\"" 'appVersion: "{{inputs.server.tag}}"' -f charts/sourcegraph-appliance/Chart.yaml
 
       - name: "update helm docs"
         cmd: ./scripts/helm-docs.sh