Remove ManiphestTaskDescriptionChangeController

Summary:
Ref T2217. This showed the text diff when you updated the description of a task, but is now obsolete.

Remove flags and methods related to rendering this pathway.

Test Plan: Clicked the fancy new "Show Details" instead.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2217

Differential Revision: https://secure.phabricator.com/D7071