cmd/go: delete TestGoBuildDashA*

cmd/go: delete TestGoBuildDashA*

The behavior of -a no longer changes depending on which kind
of branch of Go you are using (the new build cache fixed all that).
These tests are not doing anything useful (and failing).

Change-Id: I1c65120a3e05286e888951d61bca4a903e2c1158
Reviewed-on: https://go-review.googlesource.com/123575
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>