x86/MSI: drop workaround for insecure Dom0 kernels

x86/MSI: drop workaround for insecure Dom0 kernels

Considering that
- the workaround is expensive (iterating through the entire P2M space
  of a domain),
- the planned elimination of the expensiveness (by propagating the type
  change step by step to the individual P2M leaves) wouldn't address
  the IOMMU side of things (as for it to obey to the changed
  permissions the adjustments must be pushed down immediately through
  the entire tree)
- the proper solution (PHYSDEVOP_msix_prepare) should by now be
  implemented by all security conscious Dom0 kernels
remove the workaround, killing eventual guests that would be known to
become a security risk instead.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>