Remove Unsaved Changes Warning

The current unsaved changes warning was a hack, but [it will be rolled into Canvas as a feature shortly](https://help.instructure.com/entries/31780970-04-12-14-Canvas-Production-Release-Notes-Featuring-Profile-Picture-Uploader). It's annoying enough in its present incarnation that I'd rather disable it and wait for the "real" feature.

Signed-off-by: Seth Battis <SethBattis@stmarksschool.org>