iommu/amd: Don't call mmu_notifer_unregister in __unbind_pasid

This function is called only in the mn_release() path, so
there is no need to unregister the mmu_notifer here.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Tested-by: Oded Gabbay <Oded.Gabbay@amd.com>