ci: remove proof runner workflow (#11486)

The 'on: status' trigger is too broad and causes this job to trigger
for all commits. Instead KaaS (Kontrol as a Service) will post the
resulting status to slack directly, instead of through github

.github/workflows/proof-runner-notification.yml

-name: Proof Runner Deployment Status
-on:
-  status
-jobs:
-  handle-proof-runner:
-    runs-on: ubuntu-latest
-    if: >-
-      github.event.state == 'error' ||
-      github.event.state == 'failure'
-    steps:
-    - name: Generate Slack Payload
-      run: |
-        echo '{
-          "kontrol_status": "${{ github.event.state }}",
-          "ci_run_link": "${{ github.event.target_url }}"
-        }' > payload.json
-    - name: Send Status Check to Slack
-      uses: slackapi/slack-github-action@v1.26.0
-      with:
-        payload-file-path: "./payload.json"
-      env:
-        SLACK_WEBHOOK_URL: ${{ secrets.SLACK_KONTROL_WEBHOOK_URL }}
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