KVM: PPC: Book3S PR: Get rid of unused local variable

@is_mmio has never been used since introduction in
commit 2f4cf5e4 ("Add book3s.c") from 2009.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>