Remove ancient Remarkup constants from Phame and Maniphest

Summary: Ref T13661. These constants are no longer used.

Test Plan:
For a blog with a nonempty description, viewed the "Manage" page and the home page; saw properly rendered description.

Grepped for removed constants, no hits.

Maniphest Tasks: T13661

Differential Revision: https://secure.phabricator.com/D21749