ci(issue-regex-labeler): remove sync-labels option (#25377)

github/issue-labeler v3 introduced the following breaking
change: "Issue labels that do not match a regex will no longer
be removed by default unless you set sync-labels: to 1"

So we added `sync-labels: 1` to keep the previous behavior.

However, we only label new issues, not existing issues,
so the action does not need to sync or remove any labels.

.github/workflows/issue-regex-labeler.yml

@@ -12,4 +12,3 @@ jobs:
           repo-token: "${{ secrets.GITHUB_TOKEN }}"
           configuration-path: .github/issue-regex-labeler.yml
           enable-versioned-regex: 0
-          sync-labels: 1