ci: delete stale workflow

.github/workflows/stale.yml

@@ -1,17 +0,0 @@
-name: "Stale"
-
-on:
-  schedule:
-    - cron: "6 6 * * *"
-
-jobs:
-  stale:
-    runs-on: ubuntu-18.04
-    steps:
-      - uses: actions/stale@v1
-        with:
-          repo-token: ${{ secrets.GITHUB_TOKEN }}
-          stale-issue-message: 'This issue is stale because it has been open 21 days with no activity. Remove stale label or comment or this will be closed in 7 days.'
-          stale-pr-message: 'This pull request is stale because it has been open 21 days with no activity. Remove stale label or comment or this will be closed in 7 days.'
-          days-before-stale: 21
-          days-before-close: 7