Revert attempt to fix mail in differential

Summary: This fix is wrong - should be load and not get - but moreover this is actually correctly set as the reply handler is instantiated inside the DifferentialRevisionMailReceiver correctly; $this->getExclude was correct. Ref T5185.

Test Plan: this shall stop the fatal in production.

Reviewers: chad, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5185

Differential Revision: https://secure.phabricator.com/D10101