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.
Reviewed By: epriestley
Subscribers: Korvin, epriestley
Maniphest Tasks: T6179
Differential Revision: https://secure.phabricator.com/D11259