Remove "iconv" PHP extension dependency

Summary: Depends on D20069. Ref T13232. This is a very, very weak dependency and we can reasonably polyfill it.

Test Plan: Grepped for iconv in libphutil, arcanist, and Phabricator.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13232

Differential Revision: https://secure.phabricator.com/D20070