Remove the Maniphest-specific bulk job type

Summary: Depends on D18863. Ref PHI173. Ref T13025. After D18863, this job type is no longer used: the workflow uses a genric worker instead which can apply transactions to any object.

Test Plan: Grepped for callsites, found none.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13025

Differential Revision: https://secure.phabricator.com/D18864