Drop nonsense buildStatus field from Buildable

Summary:
Ref T4809. Buildables currently have buildStatus and buildableStatus. Neither are used, and no one knows why we have two.

I'm going to use buildableStatus shortly, but buildStatus is meaningless; burn it.

Test Plan: `grep`, examined similar get/set calls, created a new buildable, ran storage upgrade.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4809

Differential Revision: https://secure.phabricator.com/D8796