x86/amd-iommu: Remove iommu_flush_domain function

This iommu_flush_tlb_pde function does essentially the same.
So the iommu_flush_domain function is redundant and can be
removed.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>

diff --cc arch/x86/kernel/amd_iommu.c
Simple merge