chore: remove unused GH Workflows

.github/workflows/backport.yml

@@ -1,19 +0,0 @@
-name: Backport PR to stable
-on:
-  pull_request:
-    branches: [ develop ]
-    types: [ closed ]
-jobs:
-  release_pull_request:
-    if: github.event.pull_request.merged == true && contains(github.event.pull_request.labels.*.name, 'backport')
-    runs-on: ubuntu-latest
-    steps:
-    - name: checkout
-      uses: actions/checkout@v3
-      with:
-        fetch-depth: 0
-    - name: Backport PR by cherry-pick-ing
-      uses: Nathanmalnoury/gh-backport-action@master
-      with:
-        pr_branch: 'stable'
-        github_token: ${{ secrets.GITHUB_TOKEN }}