Remove 'setShadow' from PHUIBoxView

Summary: Removes setShadow, uses setBorder instead

Test Plan: Tested multicolumn, feed, search box, many other areas. Things look less 3d.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8192