R1:f555fbcb50db

Remove unused "MARKUP_FIELD_SUMMARY" for Phame posts

Summary: Ref T13661. This ancient "RemarkupInterface" constant has no references.

Test Plan: Grepped for references, found none. Viewed a blog, viewed a post.

Maniphest Tasks: T13661

Differential Revision: https://secure.phabricator.com/D21753