scripts: use vanilla instead of version for verity.

chromeos-base/verity now includes an impleementation to use
the new dm-verity format by setting the vanilla arg to true.
This change drops the 'version argument' for the 'vanilla'.

BUG=b/501210579
TEST=presubmit

Change-Id: Ia99653124859b20e1aaf2a2fbe4321de0e54e659
Reviewed-on: https://cos-review.googlesource.com/c/third_party/platform/crosutils/+/169606
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>