cmd/go: remove TestGoTestDetectsTestOnlyImportCycles

The error that's tested in this test is also tested in list_test_err.txt
which uses go list -test -deps. Because both commands are just loading
packages, the difference is not meaningful.

Updates #36320
Updates #17751

Change-Id: Ie712a77d64e8985dd908a1afb515ed3ecc0a9985
Reviewed-on: https://go-review.googlesource.com/c/go/+/212937


Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>