simplify asyncio staggered_race

[graingert]

Feature or enhancement

Proposal:

now that we have eager_start=False we don't need an Event() to make tasks wait until they are added to the staggered task list

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• gh-145107: simplify staggered race using eager_start=False #145108