Drop DataSource handling

Closes #29 (closed)

• Removes arkindex_worker.worker.MANUAL_SLUG
  • No workers affected in teklia/workers
• Removes ElementsWorker.get_ml_result_slug
  • Affects generic_exporter (here, here, here, here)
  • Affects horae_exporter (here and here)
  • Affects balsac_exporter (here, here and here)