Since https://golang.org/cl/38533, this validation is performed in
driverArgs.

Change-Id: I13a3ca46a1aa3197370de1095fb46ab83ea4628c
Reviewed-on: https://go-review.googlesource.com/91115Reviewed-by: Daniel Theophanes <kardianos@gmail.com>
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>