R1:114568df7a5f

Remove unused `ManiphestTaskDescriptionPreviewController` class

Summary: This class is no longer used after D6673.

Test Plan: `grep`

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11167