Drop support for parsing non-URL differential IDs from commit message.

Summary: This has been a `TODO` for a few years now... no-one should be using the old-school (non-URL) syntax anymore.

Test Plan: I'm actually not sure if it's okay to drop support for this... please verify.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9595