kvm/ppc/mpic: Eliminate mmio_mapped

We no longer need to keep track of this now that MPIC destruction
always happens either during VM destruction (after MMIO has been
destroyed) or during a failed creation (before the fd has been exposed
to userspace, and thus before the MMIO region could have been
registered).

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>