[PATCH] ppc64: prep for NUMA sparsemem rework

Remove an unused numa define and move a discontigmem specific define
inside the relevant ifdef.

I will submit a separate patch to remove them from other architectures,
but the ppc64 patches to follow depend on this.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>