hw/ppc/spapr: Remove duplicated "pseries" alias

hw/ppc/spapr: Remove duplicated "pseries" alias

The "pseries" alias is currently set twice, one time for the
pseries-2.4 machine and one time for the "pseries-2.5" machine.
To avoid confusion with the alias, let's remove the one from
the older machine class. And while we're at it, also remove
the "is_default = 0" there since the is_default variable
should be set to zero by default already.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>