Remove AphrontPanelView

Summary: Removes AphrontPanelView, and most of it's CSS - it seems some old previews still call it.

Test Plan: grep for AphrontPanelView, no callsites left. Verify CSS left is minimal needed.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7427

Differential Revision: https://secure.phabricator.com/D12004