internal/singleflight: make DoChan only return Result channel

internal/singleflight: make DoChan only return Result channel

So next CL can delete "internal/singleflight" and vendor
"golang.org/x/sync/singleflight" instead.

For #31697

Change-Id: I020da1e5a48d484637b538c010029218f5a4a744
Reviewed-on: https://go-review.googlesource.com/c/go/+/423655
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>