go/types, types2: consistently use ast/syntax.Unparen (cleanup)

go/types, types2: consistently use ast/syntax.Unparen (cleanup)

This further reduces the differences between go/types and types2.

Change-Id: I5ed0f621e1d64cd65b6a3e8eaca9926a1ccb5794
Reviewed-on: https://go-review.googlesource.com/c/go/+/562776
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>