Remove buildkite-era comment

.github/workflows/tests.yml

@@ -38,9 +38,6 @@ jobs:
     if: ${{ github.base_ref == 'develop'  || contains(github.base_ref, 'release-') }}
     runs-on: ubuntu-latest
     steps:
-      # Note: This and the script can be simplified once we drop Buildkite. See:
-      #   https://github.com/actions/checkout/issues/266#issuecomment-638346893
-      #   https://github.com/actions/checkout/issues/416
       - uses: actions/checkout@v2
         with:
           ref: ${{ github.event.pull_request.head.sha }}