Fix fatality notice casualty field duplication

www.agileplannerapp.com/boards/173808/cards/8196

this issue seems to have its roots in this refactoring:
https://github.com/alphagov/whitehall/commit/3b8145

the remove button handler script adds an additional
hidden field '_destroy' to the form with value true,
which should be sent to the server, which was not
the case. instead this extra hidden field was being
sent to the server.
Merge pull request #1862 from alphagov/fix-casualty-fields-duplication

Fix casualty fields duplication