go/types, types2: remove under(Type) in favor of Type.Underlying()

go/types, types2: remove under(Type) in favor of Type.Underlying()

As of CL 695977, under(Type) simply delegates to Type.Underlying().
This is just a cleanup.

Change-Id: I48d5fddc38560dfe485184faa6a5ff713bea74a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/712400
Commit-Queue: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>