Fix unusual use of Remarkup in Maniphest

Summary: Fixes T10234. This usage is unusual, out of date, and has some bad interactions with engines and custom rules.

Test Plan:

• Added CustomInlineCodeRule from P1129 as an extension rule.
• Put a custom <code> ... </code> block in a Maniphest task description.
• Saw fatal as described in task; applied change; saw rule work properly.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10234

Differential Revision: https://secure.phabricator.com/D15501