[dev.typeparams] go/types: use correct type parameter list in missingMethod

This is a port of CL 321232 to go/types, adjusted to add a missing
comment and to remove optional support for method type params.

Fixes #46275

Change-Id: I63fcbb669e7607876a888fca89b9064568805448
Reviewed-on: https://go-review.googlesource.com/c/go/+/324751


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>