Use ManiphestTransactionQuery directly in maniphest.gettasktransactions

Summary: Ref T2217. Nukes a LegacyQuery callsite.

Test Plan: Called method using console, verified data looks sane.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2217

Differential Revision: https://secure.phabricator.com/D7077