hw/ppc/spapr: Remove SpaprMachineClass::phb_placement callback

hw/ppc/spapr: Remove SpaprMachineClass::phb_placement callback

The SpaprMachineClass::phb_placement callback was only used by
the pseries-4.0 machine, which got removed. Remove it as now
unused, directly calling spapr_phb_placement().
Move spapr_phb_placement() definition to avoid forward declaration.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Chinmay Rath <rathc@linux.ibm.com>
Link: https://lore.kernel.org/qemu-devel/20251021084346.73671-10-philmd@linaro.org
Signed-off-by: Harsh Prateek Bora <harshpb@linux.ibm.com>