remove mathjax form main xmodule requirement and add it on problem addin...

...g

TNL-823
@waheedahmed @mlkwaqas @adampalay 
I don't think that whether or not require exists is a valid way of telling CMS vs. LMS. If you expect mathjax to be on the window object, check if it is (and when RequireJS is added to LMS, make sure that the config says that mathjax exports the right thing).