Remove PhabricatorFeedStoryManiphest and ManiphestAction

Summary:
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.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7114