Avoid double escaping in Remarkup

Summary:
These all work with plain text.

Fixes T2574.

Test Plan:
  lang=remarkup
  > [[ a | <a> ]]

  - [[ /a | <a> ]]

  //a**b**c//

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2574

Differential Revision: https://secure.phabricator.com/D5060