nixos.tests.swraid: Drop mdadm -W

This command is racy because it will return a non-zero exit code if
the array is already clean. This caused numerous random failures. It
should be unnecessary anyway. (Maybe in the past we needed this
because of #15226.)

http://hydra.nixos.org/job/nixos/release-16.03/nixos.tests.installer.swraid.i686-linux