Remove ContentFormatter from Document

[markpollack]

This came from the early days of the project and was inspired by llamaindex. In practice, the formatting of the prompt for different use cases, to include metadata or not, seems to be best handled outside the Document class.

If you have a good use case for keeping this functionality in Document, please comment.

[markpollack]

need to review the impact in textpslitter impls

[markpollack]

We had some usage of this recently, I think we should keep it, the use cases - as described in https://docs.llamaindex.ai/en/stable/module_guides/loading/documents_and_nodes/usage_documents/#customizing-embedding-metadata-text