BUG: Remove instructor already added to class instructor list from list of available instructors

When selecting the instructors for a class, an instructor who is already added to a class should not still be available in the list, which could result in multiple instances of the same instructor in the list with multiple notifications for the class. This is a bug.

Also, if you do implement this, please reverse the process so if I remove an instructor from the list, it is then again available in the list--since there doesn't seem to be a way to reorder the list.