Remnants of compatibility with python datatable library dilute code

[badasahog]

There are still old code paths that relate to shared files between this project and (I believe) a python project called datatable.

This can be seen in macros like DTPY which no longer do anything.

If compatibility is well and truly dead, I say we get rid of these.

[avimallu]

In case you need a reference, it is this one: https://github.com/h2oai/datatable/

I remember some license changes (for data.table) to facilitate the Python project, though not quite sure what the status of that one is now.

[MichaelChirico]

Per #7032 (comment), the paths have truly diverged. Maybe @st-pasha could give a final confirmation?