PCI: Remove unused and broken to_hotplug_slot()

to_hotplug_slot() is unused and wouldn't work anyway, because struct
hotplug_slot no longer contains a struct kobject (it was removed by
f46753c5 ("PCI: introduce pci_slot")).

Remove to_hotplug_slot().

[bhelgaas: changelog]
Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>