R1:f1d343111f30

Remove an unused feed story class

Summary: This class is no longer used after D7370.

Test Plan: `arc lint` and `arc unit`

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11132