Remove `SubController`

[GDYendell]

The restrictions on SubController are fairly arbitrary at this point and no logic is particularly depending on it. We should just remove it and allow a bit more flexibility in nesting controllers. For example, we could feasibly have EigerController use OdinController as a sub controller at some point, but we would also want OdinController to be the root.

BaseController can be kept for now as we will use it in #122 to restrict those from being the base controller, which would be confusing.

Acceptance Criteria

• SubController is removed

[GDYendell]

Completed in #222