Delete preference for Diffusion symbols

Summary:
We are doing a better job in 1) of D3069#3 and 2) is just dumb.
Let's see if someone notices this change.

Test Plan: /settings/panel/display/

Reviewers: nh, epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4789