[dev.typeparams] cmd/compile/internal/types2: remove disabled code related to type lists

Earlier (contract-based) versions of the generics design restricted the
kind of types that could be used in a type list - the current design does
not have these restrictions anymore. Remove the respective checking code.

Change-Id: Ia333f7aa8a9c31a92c08acbd5cadba3532a455b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/281546


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