Remove unnecessary actions/checkout in PR labeller job

Author: tianon

.github/workflows/label-prs.yml

@@ -17,7 +17,6 @@ jobs:
     name: Label
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
       - name: Apply Labels
         uses: actions/github-script@0.9.0
         with: