Remove old code in Gradebook.coffee

The removed code was first removed here: https://github.com/instructure/canvas-lms/commit/a51385bc02edcaf5540a85825abd547af485c70c, but was inadvertently added back in when we pulled in the 2014-01-18 release. My apologies for missing this in the first place.

Our version of `Gradebook.coffee` should now **only** differ from Instructure's version by changes marked with or wrapped around **`SFU MOD`** markers.