Remove icon nav entirely
Summary: Ref T10054. People, projects, and instances no longer use icon nav, so we can get rid of it in favor of profile menus.
Test Plan: Grepped for everything, looked through workboards/profiles again.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T10054
Differential Revision: https://secure.phabricator.com/D15022