ci: fix failure on lack of labeler config (#6520)

After merging https://github.com/ethereum-optimism/optimism/commit/2901c666d25b2250f51394445ae72ca7b3f31daf
which deleted the labeler config, CI now errors. This deletes the CI job
responsible for labeling, since its config is gone now.

.github/workflows/labeler.yml

----
-name: "Pull Request Labeler"
-on:
-  - pull_request_target
-
-jobs:
-  pr-labeler:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/labeler@main
-        with:
-          repo-token: "${{ secrets.GITHUB_TOKEN }}"
-          configuration-path: .github/labeler.yml