powerpc/mm: Remove very old comment in hash-4k.h

This comment talks about PTEs being 64-bits and PMD/PGD being 32-bits,
but that hasn't been true since 2005 when David Gibson implemented
4-level page tables in the commit titled "Four level pagetables for
ppc64".

Remove it.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>