R1:5914bbd806e6

Remove `*TransactionType` classes

Summary: Remove the `*TransactionType` classes and define the constants in the corresponding `*Transaction` class instead.

Test Plan: `grep`

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D13188