Implement PhabricatorDestructibleInterface for event triggers

Summary: Ref T6881. When stuff with triggers is destroyed, it should destroy the triggers.

Test Plan: Will test in Instances.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6881

Differential Revision: https://secure.phabricator.com/D11435