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