Drop hooks (now that we have a create/start split)

[wking]

With #384, I expect we will be dropping hooks on all platforms. The only uncertainty is around pre-pivot mount manipulation, and I'm not clear on why you need a hook for that. In the absence of a spec-defined run (added at one point during #384 development but removed before merge), I see no reason to keep hooks around.

Can anyone explain why we need hooks for pre-pivot mount manipulations? Or what other use-cases need hooks (vs. calling stuff between create and start)? Otherwise we should drop them to avoid
complications like #256 and #472.

[wking]

We're keeping hooks. Reasoning here.