Fix casualty fields duplication

http://www.agileplannerapp.com/boards/173808/cards/8196
fixes: https://govuk.zendesk.com/agent/tickets/872828

casualty entries on fatality notices were being duplicated because of duplicate `_destroy` inputs on the form. this was causing the 'Remove' button to not work as expected. seemingly, a result of [this refactoring](https://github.com/alphagov/whitehall/commit/3b8145).