Dr. David Alan Gilbert authored Sep 18, 2024 and Michael Tokarev committed Sep 20, 2024

The last use of sysbus_mmio_unmap was removed by
  981b1c62 ("spapr/xive: rework the mapping the KVM memory regions")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <dave@treblig.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>