Remove subprojects from the Projects UI and API

Summary:
These are currently useless and confusing (they have no application impact), and should be migrated to edges if we want to restore them in some form.

I left the actual storage so this doesn't destroy any data, it just removes all traces of this feature from the UI.

Test Plan: Looked at and edited projects.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D3183