The original function was defined in `lms/djangoapps/instructor/views/legacy.py`. In #2865 I moved this function into `lms/djangoapps/instructor/views/instructor_dashboard.py` which was incorrect; I ought to have moved it into this file (`views/api.py`). This PR removes this function from both `legacy.py` and `instructor_dashboard.py`, just having 1 definition in 1 file and all references to this function point at this one implementation.