Remove "HarbormasterBuildableTransaction::TYPE_CREATE"

Summary: Ref T13072. This transaction type has no writers and is mooted by EditEngine.

Test Plan: Grepped for transaction constant.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13072

Differential Revision: https://secure.phabricator.com/D21693