Direct Transfer - Remove old user mapping code

Everyone can contribute. Help move this issue forward while earning points, leveling up and collecting rewards.

• Close this issue

After removing the bulk_import_importer_user_mapping FF in #508945 (closed), we can remove the old user mapping code from Direct Transfer.

This issue is to remove the old user mapping code which is behind the code check Import::BulkImports::EphemeralData#importer_user_mapping_enabled?