chore(librarian): clean up owlbot files and pin image sha (#1865)

Additional clean up following
#1838

---------

Co-authored-by: Kevin Zheng <[email protected]>

Author: parthea

.github/sync-repo-settings.yaml

@@ -1,4 +1,4 @@
-image: us-central1-docker.pkg.dev/cloud-sdk-librarian-prod/images-prod/python-librarian-generator:latest
+image: us-central1-docker.pkg.dev/cloud-sdk-librarian-prod/images-prod/python-librarian-generator@sha256:39628f6e89c9cad27973b9a39a50f7052bec0435ee58c7027b4fa6b655943e31
 libraries:
   - id: google-auth
     version: 2.45.0