R1:2d15c3c06289

Tidy up ManiphestTask destruction

Summary: Now that `ManiphestTask` implements `PhabricatorApplicationTransactionInterface`, the transaction removal happens automatically.

Test Plan: Used `./bin/remove destroy` to delete a `ManiphestTask` and saw related transactions removed as well.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D13110