Removal of withdrawn predictions from the overdue predictions list

Hi! I'm an enthusiastic PredictionBook user, and while I find the user experience of PredictionBook mostly awesome, the bug where withdrawn predictions still show up in the list of unjudged predictions on your overdue predictions page (issue #48) is kind of annoying. I reckoned it would be an easy one-line fix to patch this bug, so I just went ahead and did it, and indeed it was a simple one-line fix.

Oh, and thanks for making PredictionBook! It truly is an excellent website.