go/build: remove use of package log
go/build: remove use of package log

package log does not belong here,
even for an impossible condition.

Change-Id: I6180a4f7766a8e5759db4f33d03703d0b678d0b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/241077
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

cloudflare/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,golang/go,cloudflare/go,golang/go,nawawi/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,golang/go,golang/go,cloudflare/go,cloudflare/go,golang/go