PCI: Remove pci_fixup_parent_subordinate_busnr()

The function has no effect.

If pcibios_assign_all_busses() is not set then the function does nothing.

If it is set then in pci_scan_bridge we are always in the branch where
we assign the bus numbers ourselves and the subordinate values of all
parent busses will be set to 0xff since that is what they inherited from
their parent bus and ultimately from the root bus.

Signed-off-by: Andreas Noever <andreas.noever@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>