Remove "HarbormasterBuildTransaction::TYPE_CREATE"

Summary: Ref T13072. No callers currently generate these transactions, and they probably never have. Remove them.

Test Plan: Grepped for "HarbormasterBuildTransaction::TYPE_CREATE" and "self::TYPE_CREATE" in the class, found no hits.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13072

Differential Revision: https://secure.phabricator.com/D21688