Remove HeraldDryRunAdapter

Summary: Ref T2769. This isn't a real adapter and its methods are increasingly hacky messes. Make "dry run" a first-class concept on the HeraldEngine instead and remove the adapter.

Test Plan: Ran Herald via test console and via CLI.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2769

Differential Revision: https://secure.phabricator.com/D6693