Update sync-labels to use boolean (flutter#139516)

actions/labeler#112 has been fixed, and now requires boolean to be used. 

Related infra issue that this fixes: flutter#139511

Author: drewroengoogle

.github/workflows/labeler.yml

@@ -18,5 +18,3 @@ jobs:
     steps:
     # Source available at https://github.com/actions/labeler/blob/main/README.md
     - uses: actions/labeler@8558fd74291d67161a8a78ce36a881fa63b766a9
-      with:
-        sync-labels: ''