Clean up Remarkup Code Block Headers

Summary: Removes the special background and fonts. Uses just a simple bold header.

Test Plan:
Checked out installation guide and remarkup guide in Diviner. Looks cleaner.

{F314679}

{F314680}

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6904

Differential Revision: https://secure.phabricator.com/D11869