powerpc: Remove ppc_md.remove_memory

We have an extra level of indirection on memory hot remove which is not
matched on memory hot add. Memory hotplug is book3s only, so there is
no need for it.

This also enables means remove_memory() (ie memory hot unplug) works
on powernv.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>