go/types, types2, go/ast, go/parser: remove support for type lists

This is a rough port of CL 354131 to go/* libraries, though in practice
I just tried to reconcile any places where the phrase "type list"
occurred in the source. This resulted in adjusting quite a bit more code
than initially expected, including a few lingering cases in the
compiler.

Change-Id: Ie62a9e1aeb831b73931bc4c78bbb6ccb24f53fb0
Reviewed-on: https://go-review.googlesource.com/c/go/+/359135


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>