CI/GitHub: Remove PR GitHub Action because of current limitations

GitHub Actions triggered when creating a Pull Request receive a read-only token that makes them useless when trying to do anything on GitHub.
Useful links:
- https://github.com/actions/first-interaction/issues/10
- https://github.com/actions/labeler/issues/12
- https://github.com/actions/labeler#pull-request-labeler

.github/workflows/pr-labeler.yml

@@ -1,14 +0,0 @@
-on:
-  pull_request:
-    types: [opened]
-
-jobs:
-  pr_labeler:
-    runs-on: ubuntu-latest
-    name: Pull Request Labeler
-    steps:
-    - name: Pull Request Labeler
-      id: pr-labeler
-      uses: TrinityCore/GitHub-Actions@pr-labeler-prod
-      with:
-        token: ${{ secrets.GITHUB_TOKEN }}