Remove toJSON.

Summary: As described in https://docs.google.com/a/khanacademy.org/document/d/1yhiu04vu-OsW6tdv0NPemJBNy1hETaX6cST5E_MOohY/edit#heading=h.roia7ocwoqv

Test Plan:
* make test
* at dev:9000/test.html:
  - test transformer and interactive graph
    ~ configure them
    ~ serializeQuestion
    ~ permalink and reload

Reviewers: aria

Reviewed By: aria

Subscribers: eater, alex

Differential Revision: http://phabricator.khanacademy.org/D12465