[LATER] Remove deprecated mark-committed workflow.

Summary: This workflow has been deprecated for a long time now. At some stage, it should be removed.

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9463