Remove libprotobuf dep

[jdye64]

Which issue does this PR close?

Closes #526

[jorisvandenbossche]

With datafusion-substrait now being an optional feature, that also means that the datafusion python wheels will no longer include substrait support? Is there a way to still install it with pip?

[jdye64]

With datafusion-substrait now being an optional feature, that also means that the datafusion python wheels will no longer include substrait support? Is there a way to still install it with pip?

This is a good point I had not considered. Seems like we need to make sure at release time to specify that feature should be included. I think it is fine to always include substrait in the python wheels. I think the problem was mostly for 3rd party applications that are using ADP more like an SDK. Not sure on Andy's process for a release. @andygrove is there something I might need to commit/file change to enable that for each release?