Remove one arg from CurrentQuestionHelper#previous_response_key
Remove one arg from CurrentQuestionHelper#previous_response_key

The value of `question_number` is always `nil`, so there's no need to have it
as an argument to the method.

stwalsh/smart-answers,aledelcueto/smart-answers,aledelcueto/smart-answers,alphagov/smart-answers,stwalsh/smart-answers,alphagov/smart-answers,stwalsh/smart-answers,alphagov/smart-answers,alphagov/smart-answers,stwalsh/smart-answers,aledelcueto/smart-answers,aledelcueto/smart-answers