fix: only notify Slack when merge-train PR is dequeued without being merged (#19146)

## Summary
- Skip Slack notification when a merge-train PR is dequeued due to
successful merge
- Only notify when the PR is removed from the merge queue for other
reasons (failed checks, conflicts, etc.)

## Test plan
- Merge to next, verify no notification when merge-train PR merges
successfully
- Verify notification still fires when merge-train PR is dequeued due to
failure

Author: johnathan79717

.github/workflows/merge-queue-dequeue-notify.yml

@@ -8,7 +8,7 @@ jobs:
   notify-slack:
     name: Notify Slack
     runs-on: ubuntu-latest
-    if: startsWith(github.event.pull_request.head.ref, 'merge-train/')
+    if: startsWith(github.event.pull_request.head.ref, 'merge-train/') && github.event.pull_request.merged != true
     steps:
       - name: Checkout
         uses: actions/checkout@v4