Don't delete tasks attached by freeform fields in Maniphest Tasks field

Summary: I didn't catch this issue at D3986 because we don't have DifferentialManiphestTasksFieldSpecification in field selector.

Test Plan:
Added DifferentialManiphestTasksFieldSpecification too field selector.
Wrote Refs T4 and not filled T4 to Maniphest Tasks field.

Reviewers: epriestley, 20after4

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T945

Differential Revision: https://secure.phabricator.com/D4073