Remove extent type swap code for accession CSV import

Related to #43810817.

An earlier iteration of `migrations/importers/csv.rb` had `@swap` to override mappings for a given property: https://github.com/hudmol/archivesspace/blob/fd3c981745a6a6a28e505e120dcc4db0602c11e7/migrations/importers/csv.rb#L20