Workboards - on edit, remove a task if no longer associated with workboard

Summary: Fixes T6179. This makes the interaction where users remove a task from a workboard much more pleasant.

Test Plan: Loaded up workboard for "A Project". Edited tasks and if / when I removed "A Project" they disappeared on save.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6179

Differential Revision: https://secure.phabricator.com/D11259