Remove subheader rule in remarkup

Summary: Fixes T9942, users aren't expecting this, and we can make subheader explicit later.

Test Plan: Edit Phriction Doc

Reviewers: epriestley, tycho.tatitscheff

Reviewed By: epriestley, tycho.tatitscheff

Subscribers: Korvin

Maniphest Tasks: T9942

Differential Revision: https://secure.phabricator.com/D14722