All callers of pci_do_scan_bus() are gone, so remove it.

Note that pci_do_scan_bus() was exported, so out-of-tree modules could
depend on it.

[bhelgaas: changelog]
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>