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