[dev.typeparams] go/internal/typeparams: remove the Enabled guard

[dev.typeparams] go/internal/typeparams: remove the Enabled guard

Type parameters are now always enabled. Users should guard against type
checking generic code by using the types.Config.GoVersion field.

This cleans up some differences with types2.

Change-Id: Ie3e35a549e456a90a10d6a7e158ff58653cc1394
Reviewed-on: https://go-review.googlesource.com/c/go/+/335033
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>