Remove phui-box-inner

Summary: When we removed the shadow, we no longer needed two containers.

Test Plan: Browsed Box example, a diff, a task, and other random pages. Grep for phui-box-inner, not used elsewhere.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8281