remove unneeded parameter on checkout

Author: glasnt

.github/workflows/ci.yml

@@ -10,8 +10,6 @@ jobs:
     if: contains(github.event.pull_request.labels.*.name, 'preview')
     steps:
       - uses: actions/checkout@v1
-        with:
-          ref: "refs/pull/${{ github.event.number }}/merge" # THIS IS INSECURE
       - name: Set up Python
         uses: actions/setup-python@v1
         with: