Remove support for custom logos

Summary:

• These don't fit anywhere in the new design.
• Even if we figure out how to fit them in, 220px logos definitely won't fit on the 320px iPhone screen so anyone who has a custom logo will have to rework them anyway.
• Kill it for now, and once we get the new design in and working maybe we can restore it somehow.

Test Plan: Loaded local install, no logo. Grepped for config.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1569

Differential Revision: https://secure.phabricator.com/D3101