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