Remove confusing 'while checking' note from opaque future type mismatches

[compiler-errors]

Maybe I'm just misinterpreting the wording of the note. The only value I can see in this note is that it points out where the async's opaque future is coming from, but the way it's doing it is misleading IMO.

For example:

note: while checking the return type of the `async fn`
  --> $DIR/dont-suggest-missing-await.rs:7:24
   |
LL | async fn make_u32() -> u32 {
   |                        ^^^ checked the `Output` of this `async fn`, found opaque type

We point at the type u32 in the HIR, but then say "found opaque type". We also say "while checking"... but we're typechecking a totally different function when we get this type mismatch!

r? @estebank but feel free to reassign and/or take your time reviewing this. I'd be inclined to also discuss reworking the presentation of this type mismatch to restore some of these labels in a way that makes it more clear what it's trying to point out.