Remove obsolete, nonfunctional draft auto-saving in Phriction

Summary:
Depends on D19660. Ref T5811. Ref T13077.

Long ago, if you started editing a Phriction document but didn't save it, we'd save the draft in the background as part of the preview.

D11169 updated the preview to use shared infrastructure and broke this function, since we never save drafts.

Since this doesn't work right now, I want to add another thing called "draft", and the future of this feature should be more integrated with modern drafts and EditEngine (which fixed some bugs related to versioning), just get rid of this code for the moment.

Test Plan: Edited documents. This code doesn't do anything since D11169, so no behavior changed.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13077, T5811

Differential Revision: https://secure.phabricator.com/D19661