Remove "admin page" color/styles

Summary: These haven't had any effect since the redesign, and we're moving to a more granular policy model so it probably doesn't make sense to ever restore them.

Test Plan: Grepped for "admin" and removed all relevant code.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D3710