go/parser: remove (internal) ability to disable generic code

go/parser: remove (internal) ability to disable generic code

Generics are part of the language now; there's no need anymore
to switch back to a syntax without generics. Remove the associated
machinery and adjust short tests accordingly.

Change-Id: I6b16c5c75fd9354ee87e3b9bee110f49f514565a
Reviewed-on: https://go-review.googlesource.com/c/go/+/424857
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>