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

nixos/tests/installer.nix

@@ -403,8 +403,6 @@ in {
               "mkdir /mnt/boot",
               "mount LABEL=boot /mnt/boot",
               "udevadm settle",
-              "mdadm --verbose -W /dev/md0", # wait for sync to finish; booting off an unsynced device tends to fail
-              "mdadm --verbose -W /dev/md1",
           );
         '';
       preBootCommands = ''