actions: Remove branch after auto merge

.github/workflows/merge.yml

@@ -18,3 +18,20 @@ jobs:
           MERGE_FORKS: false
           LABELS: automated-pull-request
           AUTOMERGE: automated-pull-request
+          
+  cleanup:
+    name: Cleanup
+    needs: automerge
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Extract branch name
+        shell: bash
+        run: echo "##[set-output name=branch;]$(echo $(jq --raw-output .pull_request.head.ref "$GITHUB_EVENT_PATH"))"
+        id: extract_branch
+      - name: Delete PR head branch
+        uses: dawidd6/action-delete-branch@master
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          branch: ${{ steps.extract_branch.outputs.branch }}
+          be_kind: true