chore(librarian): update image sha and clean up unused files (#149)

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

Author: parthea

packages/bigquery-magics/.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@sha256:39628f6e89c9cad27973b9a39a50f7052bec0435ee58c7027b4fa6b655943e31
 libraries:
   - id: bigquery-magics
     version: 0.10.3