iommu/amd: Remove unused struct member

Commit c805b428 ("iommu/amd: Remove amd_iommu_pd_list") removes
the global list for the allocated protection domains. The
corresponding member 'list' of the protection_domain struct is
not used anymore, so it can be removed.

Signed-off-by: Adrian Huang <ahuang12@lenovo.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>