iommu/dart: Clean up IOVA cookie crumbs

The addition of the DART driver crossed over with moving IOVA cookie
management into core code; clean up the now-unnecessary remnants here.

Acked-by: Sven Peter <sven@svenpeter.dev>
Tested-by: Sven Peter <sven@svenpeter.dev>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/edb72b4965baa8999dd70b8294785d6deebd4183.1631531973.git.robin.murphy@arm.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>