Removed unnecessary test in actionpack/test/controller/request_forgery_protection_test.rb

While working on https://github.com/rails/rails/pull/11316 I've been digging around https://github.com/rails/rails/blob/master/actionpack/test/controller/request_forgery_protection_test.rb

There is a test which I believe was written for `form_authenticity_param`. However, when I remove `form_authenticity_param`, all of the tests still pass.

This is my first Rails commit so I may be missing something obvious :)