cmd/go/internal/modfetch: remove non-hermetic test

cmd/go/internal/modfetch: remove non-hermetic test

The test for gopkg.in/yaml.v2@v2 assumes that there are
no future upstream releases. That assumption empirically
does not hold. Backporting fixes to this test is annoying,
and other gopkg.in cases are already reasonably covered,
so remove the problematic test.

Updates #28856

Change-Id: I6455baa1816ac69e02d1ad5d03b82a93e1481a17
Reviewed-on: https://go-review.googlesource.com/c/go/+/205437
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>