R1:7d1809f9352f

Remove unused `appIcon` from PHUIListItemView

Summary: This is unused and there's no way to even set it. Pretty sure it died a while ago when we switched app icons to FontAwesome.

Test Plan: Grepped for `appIcon`, found no related hits. Browsed some pages.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15074