iommu/amd: Remove amd_iommu_pd_list

This variable hold a global list of allocated protection
domains in the AMD IOMMU driver. By now this list is never
traversed anymore, so the list and the lock protecting it
can be removed.

Cc: Tom Murphy <tmurphy@arista.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>