page_size not needed for list_documents

We have three rpcs that use pagers: [ListDocuments](https://github.com/googleapis/python-firestore/blob/f10b503b0dc2b23e43495e3f0417ae2630c13d82/google/cloud/firestore_v1/collection.py#L147C48-L147C62), [ListCollectionIds](https://github.com/googleapis/python-firestore/blob/f10b503b0dc2b23e43495e3f0417ae2630c13d82/google/cloud/firestore_v1/client.py#L288), and [PartitionQuery](https://github.com/googleapis/python-firestore/blob/f10b503b0dc2b23e43495e3f0417ae2630c13d82/google/cloud/firestore_v1/query.py#L465)

Currently, onlt list_documents provides the user with a [customizable page_size arg](https://github.com/googleapis/python-firestore/blob/f10b503b0dc2b23e43495e3f0417ae2630c13d82/google/cloud/firestore_v1/collection.py#L124). But all 3 rpcs abstract the pages from the users, and provide them with a flat generator over the underlying resources, so it's debatable whether the page_size argument even makes sense to provide.

To be consistent, we should either remove it from list_documents, or provide it in all three