Remove extraneous argument to mouse playback. Fixes Mouse Interaction talkthrough.

Summary: During the recent refactor the name argument was removed to the mouse handlers, causing the `x` argument to become `y` and `y` to become `undefined`. This fixes that.

Test Plan: The Mouse Interaction talkthrough should now work!

Reviewers: brian, pamela

Reviewed By: pamela

Subscribers: #cs

Differential Revision: http://phabricator.khanacademy.org/D13787