cmd/compile/internal/types2: always parse tests assuming generic code

We don't need to distinguish anymore as this is the new default.
This removes the need to prefix test package names with "generic_".

Change-Id: If9eaa0a5cffcd19deb529aca6798206f2d396ca1
Reviewed-on: https://go-review.googlesource.com/c/go/+/361408


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>