Remove PHID_TYPE_ACMT

Summary: Ref T4896. This was used by the old audit comment storage, which is now defunct.

Test Plan: Grepped for callsites in the codebase.

Reviewers: joshuaspence, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4896

Differential Revision: https://secure.phabricator.com/D10152