powerpc/mm: Remove PPC_MM_SLICES #ifdef for book3s64

Book3s64 always have PPC_MM_SLICES enabled. So remove the unncessary #ifdef

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>