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