x86/PCI: Remove mp_bus_to_node[], set_mp_bus_to_node(), get_mp_bus_to_node()

There are no callers of get_mp_bus_to_node(), so we no longer need
mp_bus_to_node[], get_mp_bus_to_node(), or set_mp_bus_to_node().
This removes them.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>