R1:94f438f6a6e5

Remove unused `PhrictionDocumentPreviewController` class

Summary: This class is no longer required after D6675.

Test Plan: `grep`

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11169