dillon1000/react

Remove `flushDiscreteUpdates` from end of event (#21223)

We don't need this anymore because we flush in a microtask.

This should allow us to remove the logic in the event system that
tracks nested event dispatches.

I added a test to confirm that nested event dispatches don't triggger
a synchronous flush, like they would if we wrapped them `flushSync`. It
already passed; I added it to prevent a regression.