go/types, types2: unexport NewTypeList

go/types, types2: unexport NewTypeList

NewTypeList was not part of the go/types API proposal, and was left in
by accident. It also shouldn't be necessary, so remove it.

Updates #47916

Change-Id: I4db3ccf036ccfb708ecf2c176ea4921fe68089a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/369475
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>