Hide points on transformable lines

Test Plan:
Add a transformer
 * Set the type to lines or multiple lines
 * Make sure that the lines are still editable in the editor graph
 * Make sure that the points don't appear on the lines in the
   transformer graphs
 * Check that answering the question still works

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D5795