chore(librarian): remove obsolete action to update the root changelog (#14815)

Updating the root changelog is handled by the following code. The github
workflow `updatechangelog.yml` is no longer needed.


https://github.com/googleapis/google-cloud-python/blob/2b3912f9b4935c0e49d527e4f754a31e5e88aca1/.generator/cli.py#L282-L286


https://github.com/googleapis/google-cloud-python/blob/2b3912f9b4935c0e49d527e4f754a31e5e88aca1/.generator/cli.py#L1444-L1450

Author: parthea