Remove payments sprite

Summary: Removes unused payments sprite and code, also some unused conpherence generated images. We use images in login (and could use FontAwesome, maybe).

Test Plan: grep codebase, pull up uiexamples icons page

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11025