cmd/go: fix clang test break as a result of golang.org/cl/127495

golang.org/cl/127495 incorrectly tested against environment specifc
details that do not form part of the script test conditions. This broke
the clang build.

Fix by removing the specific check; the existing checks are sufficient.

Change-Id: Ic6ec873df9343c809968a08dd676e210046da5fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/175179


Run-TryBot: Paul Jolly <paul@myitcv.org.uk>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>