fix(sync-files): remove backport.yaml from source

Signed-off-by: Fumiya Watanabe <rej55.g@gmail.com>

Author: rej55

.github/sync-files.yaml

@@ -10,7 +10,6 @@
     - source: .github/ISSUE_TEMPLATE/task.yaml
     - source: .github/pull_request_template.md
     - source: .github/dependabot.yaml
-    - source: .github/workflows/backport.yaml
     - source: .github/stale.yml
     - source: .github/workflows/comment-on-pr.yaml
     - source: .github/workflows/pre-commit.yaml