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 This usage is unusual, out of date, and has some bad interactions with engines and custom rules.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