Remove ManiphestLegacyTransactionQuery

Summary: Ref T2217. No remaining callsites. Also get rid of some methods on ManiphestTransaction that nothing calls anymore.

Test Plan: `grep`, looked at tasks.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2217

Differential Revision: https://secure.phabricator.com/D7079