remove backport action, not used

Signed-off-by: dartcafe <github@dartcafe.de>

.github/workflows/backport.yml

@@ -1,16 +0,0 @@
-name: Backport
-on:
-  pull_request:
-    types:
-      - closed
-      - labeled
-
-jobs:
-  backport:
-    runs-on: ubuntu-18.04
-    name: Backport
-    steps:
-      - name: Backport
-        uses: tibdex/backport@v1
-        with:
-          github_token: ${{ secrets.GITHUB_TOKEN }}