On what date was Stauffer Chemical Company (Lemoyne Plant), which was listed on 09/21/1984, deleted?
CREATE TABLE "superfund_sites" (
  "cerclis_id" text,
  "name" text,
  "county" text,
  "proposed" text,
  "listed" text,
  "construction_completed" text,
  "partially_deleted" text,
  "deleted" text
);
SELECT "deleted" FROM "superfund_sites" WHERE "listed"='09/21/1984' AND "name"='stauffer chemical company (lemoyne plant)';