Remove Interactive Number Line Widget

Summary: Remove the old, unnecessary interactive number line widget since we no longer use it anywhere on the site (I went through and replaced them with Number Line widgets).

Test Plan: Search items.json and make sure there are no interactive-number-line widgets. Also check that nothing else is broken?

Reviewers: aria

Reviewed By: aria

Differential Revision: http://phabricator.khanacademy.org/D13634