MDL-28075 question editing: sort out nav bar when editing in a quiz

If you edited a question from the Edit quiz page in a quiz, the navigation
bar was far too long. This was due to some odd code that was trying too
hard to be clever.

Instead, editing a question is now always treated as being logically within
the question bank for the purposes of navigation. That is, even if the
returnurl will end up taking you back to where you where when you have finished.
MDL-28075 question editing: tidy code

Remove an unnecessary else (the if, and else if, always end in a redirect)
and unindent that code.