Remove ManiphestTransaction

Summary: Ref T2217. The preview had the last callsite, nuke it.

Test Plan: Used preview. Grepped for ManiphestTransaction(, ManiphestTransaction::, 'ManiphestTransaction', "ManiphestTransaction".

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2217

Differential Revision: https://secure.phabricator.com/D7092