go/types, types2: remove references to under function in comments
Follow-up on CL 712400 which removed the under function.
Change-Id: I253c8adbbaa058150f26e311e37b4c1644b6554d
Reviewed-on: https://go-review.googlesource.com/c/go/+/712520 Reviewed-by: Mark Freeman <markfreeman@google.com> Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>