removing students from a class should also remove them from the section

[lihelennn]

currently, removing a student from a class only removes them from the global section.
removing a student from a class should also cause that student to be removed from all sections they're in.

code reference:

nb/routes/classes.js

Line 326 in 1d33494

utils.removeStudent(req.params.courseid, req.params.userid)

nb/models/utils.js

Line 142 in 1d33494

removeStudent: function(classId, userId){