dillon1000/react

Simplify `discreteUpdates` (#21773)

Now that discrete updates are flushed synchronously in a microtask,
the `discreteUpdates` method used by our event system is only a
optimization to save us from having to check `window.event.type` on
every update. So we should be able to remove the extra logic.

Assuming this lands successfully, we can remove `batchedEventUpdates`
and probably inline `discreteUpdates` into the renderer, like we do
for continuous updates.