powerpc/hugetlbpage: Rmove unhelpful HUGEPD_*_SHIFT macros

The HUGEPD_*_SHIFT macros are always defined to be PGDIR_SHIFT and
PUD_SHIFT, and have to have those values to work properly.  They once used
to have different values, but that was really only because they were used
to mean different things in different contexts.

6fa50483 "powerpc/mm/hugetlb: initialize the pagetable cache correctly for
hugetlb" removed that double meaning, but left the now useless constants.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>