ARM/PCI: Remove unused pcibios_add_bus() and pcibios_remove_bus()

There are no users of the struct hw_pci.add_bus() or .remove_bus() methods,
so remove the pointers from hw_pci.  That makes pcibios_add_bus() and
pcibios_remove_bus() themselves superfluous, so remove them as well.

[bhelgaas: changelog]
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>