Remove Octicons refs
Summary: As far as I can tell, the Octicons were only used for a couple of weeks in June 2017, between rP83a89166 and rPcdeba0f85b.
Test Plan: `git log -i -G octicon`
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D26784