[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