R1:0558d53273e5

Convert maniphest to use standard fields

Summary: Ref T3794. Drop auxiliary field, use standard field.

Test Plan: Performed migration, field seemed to survive it intact. Edited and viewed tasks.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3794

Differential Revision: https://secure.phabricator.com/D7036