Remove "PhabricatorEventType::TYPE_DIFFUSION_LOOKUPUSER" event

Summary: Ref T13444. This is an ancient event and part of the old event system. It is not likely to be in use anymore, and repository identities should generally replace it nowadays anyway.

Test Plan: Grepped for constant and related methods, no longer found any hits.

Maniphest Tasks: T13444

Differential Revision: https://secure.phabricator.com/D20909