R1:e33f5b785e73

Remove unused `phabricator_on_relative_date` function

Summary: This function isn't used.

Test Plan: `grep`

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11190