iommu/amd: Remove stale amd_iommu_unmap_flush usage

Remove the new use of the variable introduced in the AMD driver branch.
The variable was removed already in the iommu core branch, causing build
errors when the brances are merged.

Cc: Nadav Amit <namit@vmware.com>
Cc: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Link: https://lore.kernel.org/r/20210802150643.3634-1-joro@8bytes.org