Fix delete merged branch workflow (opensearch-project#693)

Signed-off-by: Prudhvi Godithi <[email protected]>

.github/workflows/delete_backport_branch.yml

@@ -7,9 +7,16 @@ on:
 jobs:
   delete-branch:
     runs-on: ubuntu-latest
-    if: startsWith(github.event.pull_request.head.ref,'backport/')
+    permissions:
+      contents: write
+    if: github.repository == 'opensearch-project/job-scheduler' && startsWith(github.event.pull_request.head.ref,'backport/')
     steps:
-      - name: Delete merged branch
-        uses: SvanBoxel/delete-merged-branch@main
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      - name: Delete branch
+        uses: actions/github-script@v5
+        with:
+          script: |
+            github.rest.git.deleteRef({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              ref: `heads/${context.payload.pull_request.head.ref}`,
+            })