PCI: hotplug: Drop superfluous pci_hotplug_slot_list

PCI: hotplug: Drop superfluous pci_hotplug_slot_list

The PCI hotplug core keeps a list of all registered slots.  Its sole
purpose is to WARN() on slot removal if another slot is using the same
name.

But this can never happen because already on slot creation, an error is
returned and multiple messages are emitted if a slot's name is
duplicated:

  pci_hp_register()
    __pci_hp_register()
      __pci_hp_initialize()
        pci_create_slot()
          kobject_init_and_add()
            kobject_add_varg()
              kobject_add_internal()
                create_dir()
                  sysfs_create_dir_ns()
                    kernfs_create_dir_ns()
                      sysfs_warn_dup()
                        pr_warn("cannot create duplicate filename ...")
                pr_err("%s failed for %s with -EEXIST, ...");

Drop the superfluous list.

Link: https://lore.kernel.org/r/603735bc50eb370bc7f1c358441ac671360bab25.1740501868.git.lukas@wunner.de
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>