Remove flushDiscreteUpdates, discreteUpdates and syncUpdates

[sebmarkbage]

We're simplifying discrete by merging it with sync. As such flushSync() can be used to flush already pending discrete work. flushSync(() => ...) replaces discreteUpdates. syncUpdates could in theory be used to enter the discrete phase in a non-discrete event but might as well use flushSync(() => ...) then to simplify the API surface area.

Currently flushSync errors if nested. We might need to relax that and always flush on the inner one. Let's see if we hit it.

I'm going to remove flushControlled as well. I need to see if I can just use flushSync for that. In Draft's case I might be able to just let the new heuristics take over and make flushControlled a noop.

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[acdlite]

Is it intentional that you didn't actually remove the flushDiscreteUpdates implementation? Is the idea to remove the API only and then clean up the implementation later?

[sebmarkbage]

The implementation is still used by the event system. I'll need to implement the new model and then remove it.

[trueadm]

Looks good to me.