Reverts "Update sync-labels to use boolean" (#139533)

Reverts flutter/flutter#139516
Initiated by: godofredoc
This change reverts the following previous change:
Original Description:
https://github.com/actions/labeler/issues/112 has been fixed, and now requires boolean to be used. 

Related infra issue that this fixes: https://github.com/flutter/flutter/issues/139511

.github/workflows/labeler.yml

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