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