Remove RetrieveDataImport

When we renamed DataImport to Process, we kept a RetrieveDataImport endpoint (see comment) for backwards compatibility with older workers that could not be updated to base-worker ≥0.3.0 (teklia/workers/base-worker#151).

With the various updates that occurred during this time, particularly with the new Ponos authentication, it's safe to say that any worker using base-worker < 0.3.0 is completely dead. @schneider-y says he would have been fired if there were any of those old workers remaining. So we can now nuke RetrieveDataImport for good.