simplify Search#countries model (#27)
simplify Search#countries model (#27)

`#countries` method does not need to accept a parameter as it’ll always search `GeographicalArea`s
bitzesty/trade-tariff-frontend,bitzesty/trade-tariff-frontend,alphagov/trade-tariff-frontend,alphagov/trade-tariff-frontend,alphagov/trade-tariff-frontend,alphagov/trade-tariff-frontend,bitzesty/trade-tariff-frontend,bitzesty/trade-tariff-frontend