powerpc/mm: Simplify the code dropping 4-level table #ifdef

Simplify the code by dropping 4-level page table #ifdef. We are always
4-level now.

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