Drop unofficial substrait JSON API

[benbellick]

This library contains functions which expose JSON APIs for substrait. However, they are actually just thin wrappers around underlying calls to google.protobuf.json_format. To be consistent with the substrait specification, lets remove functions which publicly expose json-handling functions. This makes the library consistent with the others in this ecosystem (e.g. substrait-go, substrait-java). As a reminder, JSON is not an official text representation of plans.

Nothing prohibits users from leveraging google.protobuf.json_format directly, and perhaps it is even encouraged. (We may want to use it ourselves for testing infrastructure.) That being said, the substrait community doesn't claim to manage that representation.

[benbellick]

If there is strong desire to have this remain available, I can remove it from this pull request #100.