Add trigger rule to remove projects from tasks

Summary: Ref T13269. Same as D20379 with the polarity reversed.

Test Plan: Added some triggers, removed some projects, observed expected results.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T13269

Differential Revision: https://secure.phabricator.com/D20390