ci: simplify backport workflow

.github/workflows/backport.yml

@@ -8,15 +8,9 @@ jobs:
       contents: write
       pull-requests: write
     name: Backport Pull Request
-    if: >
-      github.repository_owner == 'neovim' && (
-        github.event_name == 'pull_request_target' &&
-        github.event.pull_request.merged
-      )
+    if: github.event.pull_request.merged
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
-        with:
-          ref: ${{ github.event.pull_request.head.sha }}
       - name: Create backport PRs
         uses: korthout/backport-action@v1