cmd/go: remove user-content from doc strings in cgo ASTs.

Thank you to RyotaK (https://ryotak.net) of GMO Flatt Security Inc. for reporting this issue.

Updates golang/go#76697
Fixes CVE-2025-61732

Change-Id: I1121502f1bf1e91309eb4bd41cc3a09c39366d36
Reviewed-on: https://go-review.googlesource.com/c/go/+/734220
Reviewed-by: Agustin Hernandez <garisol1982@gmail.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>