go/types, types2: remove global goVersion flag (cleanup)

go/types, types2: remove global goVersion flag (cleanup)

Now that we have an easy mechanism to set the Go version with a comment
line directly in test files, we don't need this flag anymore.

Change-Id: Ic1f1a6fdf2c6c692512bff49650916ec43645aee
Reviewed-on: https://go-review.googlesource.com/c/go/+/444275
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>