iommu/amd: Remove AMD_IOMMU_STATS

Commit e85e8f69 ("iommu/amd: Remove statistics code")
removed that configuration.

Also remove function definition (suggested by Joerg Roedel)

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Joerg Roedel <jroedel@suse.de>