Remove unused PhabricatorFeedStory subclasses
Summary: Remove a bunch of unused PhabricatorFeedStory subclasses.
Test Plan: grep
Reviewers: epriestley, Blessed Reviewers
Reviewed By: epriestley, Blessed Reviewers
Subscribers: btrahan, Korvin, epriestley
Maniphest Tasks: T5655
Differential Revision: https://secure.phabricator.com/D11674