Removed all instances of getIconURI

Summary: Fixes T11541. PhabricatorApplication::getIconURI() has been returning only null for a while (I assume in preparation to remove it). I removed the method and all the remaining call sites.

Test Plan: Removed the method and then clicked around. Things didn't explode!

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, yelirekim

Maniphest Tasks: T11541

Differential Revision: https://secure.phabricator.com/D16470