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