[POWERPC] Remove a couple of unused definitions from pgtable_32.c

In arch/powerpc/mm/pgtable_32.c, the variable io_bat_index and the
macro is_power_of_4() no longer have any users.  This removes them.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>