Remove temporary Admin::EditionRoutesHelper#document_instance_route.
Remove temporary Admin::EditionRoutesHelper#document_instance_route.

This was introduce in 4e007fd211eb6d73c6f33852a599b3b7c7dcd8b3 as a
temporary method to help me make more granular commits. It has now
served its purpose and can be removed.

alphagov/whitehall,robinwhittleton/whitehall,alphagov/whitehall,hotvulcan/whitehall,YOTOV-LIMITED/whitehall,ggoral/whitehall,ggoral/whitehall,robinwhittleton/whitehall,robinwhittleton/whitehall,YOTOV-LIMITED/whitehall,alphagov/whitehall,YOTOV-LIMITED/whitehall,hotvulcan/whitehall,hotvulcan/whitehall,YOTOV-LIMITED/whitehall,askl56/whitehall,askl56/whitehall,alphagov/whitehall,ggoral/whitehall,askl56/whitehall,hotvulcan/whitehall,robinwhittleton/whitehall,askl56/whitehall,ggoral/whitehall