go/types: remove special handling for .go1 files in tests

go/types: remove special handling for .go1 files in tests

All but local tests are now shared and reside in internal/types;
and there are no .go1 files anymore.

Change-Id: I3f8374e85639348de3cb8b568a7a05df5f9c2a38
Reviewed-on: https://go-review.googlesource.com/c/go/+/444276
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>