Un-implement MarkupInterface from Mocks and Images in Pholio

Summary: Depends on D19911. Ref T11351. MarkupInterface has mostly been replaced with PHUIRemarkupView, and isn't really doing anything for us here. Get rid of it to simplify the code.

Test Plan: Viewed various mocks with descriptions and image descriptions, saw remarkup presented properly.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T11351

Differential Revision: https://secure.phabricator.com/D19912