[dev.typeparams] go/ast: remove the typeparams build constraint

This CL removes the typeparams build constraint guarding changes to the
go/ast and go/types APIs. Notably it does not remove all indirection
added to hide the type parameter API: the go/internal/typeparams
package is not yet deleted, nor have go/parser or go/types been updated
to access type parameter data directly. This will be done in a follow-up
CL; the intent of this CL is to make it easier to support the new type
set syntax, and to experiment with different AST APIs.

Change-Id: I13ea0285752991b87b3aead1d1371e1f3f817b1a
Reviewed-on: https://go-review.googlesource.com/c/go/+/325689


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>