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