Remove white app icons

Summary: Remove white app icons, no longer in use as far as grep/memory serve. These were for list hover states.

Test Plan: Rebuild sprites, celerity. Grep for appIcon use (only feed). Verify all action lists are driven by FontAwesome.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin, hach-que

Differential Revision: https://secure.phabricator.com/D9078