Remove comment plugins from Phame

Summary: Cleaning up house, may revisit in a v2. Removes ability to set Disqus or Facebook comments as comment system on Phame Posts.

Test Plan: Create blog, create post, edit blog, view live pages.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: btrahan, Korvin

Maniphest Tasks: T9746

Differential Revision: https://secure.phabricator.com/D14448