Remove button bar sprite images

Summary: These were converted to FontAwesome, but the images remained. Also removing a stray apps sprite (unused).

Test Plan: Visit button bar UIExample, Calendar. No issues. Grep codebase.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11026