Remove method call receiver special casing in unused_async lint

[y21]

Fixes the false positive mentioned in #13466 (comment).

The false negative in the OP would be nice to fix too, but I'd rather do that in a separate PR because it's much more involved

Before this change, the unused_async lint would check if the async fn is also used anywhere and avoid linting if so. The exception is if the async function is immediately called, because the returned future handling can be easily removed (and also if we don't have some exceptions then the lint wouldn't trigger anywhere) or if it's a method call receiver.

I'm not exactly sure why I implemented that special casing for method call receivers in #11200, but it doesn't make much sense in hindsight imo. Especially given that method calls are essentially equivalent to function calls with the receiver as the first argument, which was the primary motivation for not linting in the first place (async fn passed to another function, like axum::get(handler) where handler has to be an async fn).

changelog: none

[rustbot]

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[llogiq]

Looks good to me. Thank you!

@bors r+

[bors]

📌 Commit d24a631 has been approved by llogiq

It is now in the queue for this repository.

[bors]

⌛ Testing commit d24a631 with merge 897f0e4...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: llogiq
Pushing 897f0e4 to master...