subkatex
Remove support for summative exercises

Summary: Since 5512461f742fd9e8700983a81b298cd5048812b4, we don't need to support summatives any more, so let's remove this code for simplicity.

Test Plan: Loaded absolute_value_equations locally and in webapp successfully.

Reviewers: brianmerlob, eater

Reviewed By: eater

Differential Revision: http://phabricator.khanacademy.org/D6142