chore(librarian): update image sha and clean up unused files (#980)

This PR updates the SHA for the librarian language image to match the
one
[here](https://github.com/googleapis/google-cloud-python/blob/2feb74032fd9c5cc7eaf6072ab03e9e8397bd434/.librarian/state.yaml#L1C1-L1C170).

Owlbot related files are also cleaned up

---------

Co-authored-by: ohmayr <[email protected]>

Author: parthea

.github/auto-approve.yml

@@ -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:c8612d3fffb3f6a32353b2d1abd16b61e87811866f7ec9d65b59b02eb452a620
 libraries:
   - id: pandas-gbq
     version: 0.30.0