ppc: remove all unused CPU definitions

Remove *all* unused CPU definitions as indicated by compile-time
`#if 0` constructs.

Signed-off-by: John Snow <jsnow@redhat.com>
[dwg: Removed some additional now-useless comments]
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>