[dev.typeparams] go/types: remove typeparams wrappers and aliases

These wrappers and aliases were necessary when we had different versions
of the API depending on the typeparams build constraint, but now they're
just boilerplate. Remove them.

Notably the documentation for types.Info is restored.

Change-Id: I5363d1d5df47649c2641c03328dfba45b1456278
Reviewed-on: https://go-review.googlesource.com/c/go/+/334895


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>