remove big unused assets (#2122)

Doing this now because I'm trying to stretch a little more life out of our existing Jenkins machine.   Eventually will have to resize the disk, but that's a [hassle][1] that I'd rather not bother with just now.
  [1]: http://alestic.com/2010/02/ec2-resize-running-ebs-root 

I looked for big things and found two:
- offline/static/mathjax - 16MB
- main/courses/static/graphics/videos/nlp - 42MB

Could do more later but this is sufficient for now.