[dev.typeparams] cmd/compile/internal/types2: remove Named.SetTArgs

Calling SetTArgs without substituting can leave the type in incoherent
state, so we should avoid exposing this API unless necessary. Since it
is currently not used by the importer(s), it is probably not necessary
to expose for 1.18, so remove it.

Change-Id: I06bd7b5bbfacd3c65e2e66a9d5980f20cd1c10c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/341290


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>