cmd/compile/internal/syntax, types2: remove ability to handle type lists

cmd/compile/internal/syntax, types2: remove ability to handle type lists

The type set notation has been accepted a while ago.
We're not going back to supporting the original
type list notation. Remove support for it in the
parser and type checker.

Change-Id: I860651f80b89fa43a3a5a2a02cf823ec0dae583c
Reviewed-on: https://go-review.googlesource.com/c/go/+/354131
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>