Remove worker version FKs from ML results

Closes #1904 (closed)

• arkindex generate_missing_worker_runs has been merged back into the migrations, since the removed fields means it can no longer work properly outside of a migration.
• The new migrations take less than two minutes in total on my machine, even with millions of elements. The results in preprod will likely be surprising too. I still split them into one migration per model just in case.
• This removal enables an extra optimization in the worker results deletion that can avoid using one extra index when deleting with any filter set other than a worker run filter.