Remove another old remarkup engine callsite in Config

Summary: Ref T13189. Summaries do not appear to be meaningfully rendered with Remarkup so just drop the engine. See D19610 for the previous change in this vein.

Test Plan: Viewed config list with option summaries.

Reviewers: amckinley

Maniphest Tasks: T13189

Differential Revision: https://secure.phabricator.com/D19612