ref!: Remove flushAsync option everywhere

[Luca Forstner (lforst)]

Ref braintrustdata/braintrust-sdk-javascript#1841

flushAsync grinded my gears a bit a) as a user b) as a maintainer. Why?:

• As a user, it added a quite intransparent API that is hard to wrap your head around.
• As a user, it added a generic type to otherwise very non-generic APIs that made the LSP types incredibly hard to navigate
• As a maintainer, it required a generic type to be passed around to literally everywhere
• As a maintainer, it required us to have a weird flushAsync option to every tracing API

In reality, what you can do instead, and which is very simple to understand, is simply do await flush() or await logger.flush() right after a span is ended. Not hard, very few if no drawbacks.