Remove dead Maniphest field config from default.conf.php

Summary: Some day we'll just delete this entirely.

Test Plan: loaded Phabricator

Reviewers: zeeg, btrahan

Reviewed By: zeeg

CC: aran, mbishopim3

Differential Revision: https://secure.phabricator.com/D7044
Merge branch 'master' of github.com:facebook/phabricator