Undo PREFIX/SUFFIX in exam list template

@jbau or @sefk please.

@kluo pointed out that the use of ```{% with PREFIX= ... %}``` in the exams/list.html template breaks the url reversing. It's not clear to me why this should be the case, and I'm not sure what happened when I tested it the other night - I certainly remember it working. But in the meantime, let's back that out so the exams pages still load properly. Maybe I can tidy up the ```{% url %}}``` calls later.