cmd/go: delete useless env var setting in test
cmd/go: delete useless env var setting in test

The correct spelling is asyncpreemptoff=1,
but clearly this test doesn't need it.

Change-Id: Ic69cf5be04bf90d357a6e4d8ae3f9011f1779922
Reviewed-on: https://go-review.googlesource.com/c/go/+/430575
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Auto-Submit: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go