mn10300/PCI: Remove useless pcibios_last_bus

pcibios_last_bus was apparently copied from x86.  On mn10300, it is
statically initialized to -1 and may be set with the "pci=lastbus=<X>"
boot option, but it is never tested.  This patch removes everything
related to pcibios_last_bus.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>