Simplify notification code

Summary:
Currently we have two different feed story classes, one for notifications and one for feed stories. However, we never actually do anything different with them -- the notification is always the same as the feed story, just shown differently. Delete the notification special case to reduce the amount of code we have supporting feed and notifications.

This is a precursor to @chad's notification designs.

Test Plan: Viewed notifications and feed, saw exactly the same result before and after the patch (but less, simpler code).

Reviewers: btrahan

Reviewed By: btrahan

CC: chad, aran

Differential Revision: https://secure.phabricator.com/D4114