R1:99138049320c

Remove some unused images

Summary:
- Remove `app_audit.png`, which is now part of the sprite (it was held back transitionally to make sure things worked).
- Remove `sprite.png`, which is the old button sprite. The only remaining use was on the home page, which I replaced with box shadows to achieve a similar effect. These buttons should probably go away at some point anyway.

Test Plan: {F25768}

Reviewers: chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D4028