Remove PhabricatorFeedStoryManiphest and ManiphestAction
I'll hold this for a couple weeks.
These classes are now only used to render legacy feed stories. I don't plan to migrate the stories since I don't think they're particularly valuable, and migrating them would be complex and time consuming.
With these classes removed, legacy Maniphest feed stories simply vanish from feed.
Test Plan: grep, viewed feed, verified it worked but omitted old-style stories.
Reviewed By: btrahan
Differential Revision: https://secure.phabricator.com/D7114