Decouple forms from page callback via removal of arg(...) from quiz.admin.inc

You cannot currently use the quiz questions form outside of the context of the built-in page callback, because of some very old code referring to arg().

This is in the same spirit as issue #2464087: Updating AJAX quiz to utilize ajax framework and changing the use of args(), and the quiz 5.0 goal on the bullet point "*Improved DX, coding standards/style, The Drupal Way!™".

I can see more arg() stuff elsewhere, but I'll leave those to the other issue.