Remove unused workflow

Sadly this one can't work for PRs created from forks.

.github/workflows/api-changes.yml

@@ -1,32 +0,0 @@
-name: Notify when API changes are merged
-
-on:
-  pull_request:
-    branches: [ "master" ]
-    types:
-      - closed
-
-jobs:
-  if_merged:
-    if: github.event.pull_request.merged == true && contains(github.event.pull_request.labels.*.name, 'api changes')
-    runs-on: ubuntu-latest
-    steps:
-    - run: |
-        echo The PR was merged
-    - name: Send Slack notification
-      env:
-        SLACK_WEBHOOK_URL: ${{ secrets.SLACK_WEBHOOK_URL }}
-      uses: 8398a7/action-slack@v3
-      with:
-        status: custom
-        fields: repo,commit,event,action,workflow,ref,url
-        custom_payload: |
-          {
-            "attachments": [
-              {
-                "color": "${{ job.status }}",
-                "title": "${{ github.actor }} ${{ github.event.pull_request && github.event.pull_request.merged == true && 'merged a pull request' || github.event.pull_request && 'opened a pull request' || github.event_name == 'push' && 'pushed new changes' || github.event_name == 'create' && 'created a new branch or tag' || github.event_name == 'delete' && 'deleted a branch or tag' || github.event_name == 'issues' && github.event.action || 'triggered an event' }}",
-                "text": ":tada: Event details:\n\n- Repository: ${{ github.repository }}\n- Branch: ${{ github.ref }}\n- Message: ${{ github.event_name == 'push' && github.event.head_commit.message || github.event_name == 'issues' && github.event.issue.title || github.event_name == 'pull_request' && github.event.pull_request.title || '' }}\n- :globe_with_meridians: Link: ${{ github.event.pull_request.html_url || github.event.issue.html_url || github.event.repository.html_url }}"
-              }
-            ]
-          }