iommu: Remove group_mf

The iommu=group_mf is really no longer needed with the addition of ACS
support in IOMMU drivers creating groups.  Most multifunction devices
will now be grouped already.  If a device has gone to the trouble of
exposing ACS, trust that it works.  We can use the device specific ACS
function for fixing devices we trust individually.  This largely
reverts bcb71abe.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>