minor: remove custom extract_ok! macro

[ctsk]

When reading through aggregation code, I saw that the extract_ok! macro seems to do the same thing as the ? operator in this context.

/// Extracts a successful Ok(_) or returns Poll::Ready(Some(Err(e))) with errorsmacro_rules! extract_ok {($RES: expr) => {{match $RES {Ok(v) => v,Err(e) => returnPoll::Ready(Some(Err(e))),}}};}

I've replaced the usages of extract_ok with ? in this PR and removed the macro.

[ctsk]

@alamb I see that you've previously attempted something similar in #7025 - but a larger refactoring is not (no longer?) necessary to drop extract_ok.

[alamb]

Makes sense to me -- thank you @ctsk@mbrobbel and @xudong963
I think maybe when this was first written Poll didn't support the ? operator (or maybe I just didn't understand it)