frv/PCI: Remove pcibios_last_bus

pcibios_last_bus was apparently copied from x86.  On FR-V, it is
statically initialized to -1 and never changed unless the user boots
with "pci=lastbus=<X>".  I doubt that option is used on FR-V, so this
patch removes all the code related to pcibios_last_bus.

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