R1:b5bf8e998d53

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