Remove `datafusion.proto` and use the `datafusion-proto` api for logical plan serde

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
This repo contains an out-of-date copy of the `datafusion.proto` from the `arrow-datafusion` repo

**Describe the solution you'd like**
Once https://github.com/apache/arrow-datafusion/issues/2630 is implemented, we should leverage this crate for serializing logical plans.

**Describe alternatives you've considered**
None

**Additional context**
None

Start by locating the repository's out-of-date datafusion.proto copy and read the referenced arrow-datafusion issue 2630 to confirm the prerequisite API. Remove the copy and switch logical-plan serialization to the datafusion-proto API; done when the repository no longer depends on its duplicate proto and the affected serialization flow still works.

Written by the indexing model from the issue text.