Remove "HarbormasterBuildCommand"

Summary: Ref T13072. Update the last few constant references to this class and remove it.

Test Plan: Grepped for "HarbormasterBuildCommand", got no hits.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13072

Differential Revision: https://secure.phabricator.com/D21692