Remove ManiphestTaskExtensions

Summary: Ref T418. Maniphest has an obsolete class-based field selector. Replace it with CustomField-based selectors, which use the nice config UI and are generally way easier to use.

Test Plan: Added custom fields; edited and viewed custom fields on tasks. Everything worked as expected.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T418

Differential Revision: https://secure.phabricator.com/D6998