ACPI / PCI: remove stale list_head in struct acpi_prt_entry

list_head "list" in struct acpi_prt_entry was used to connect
_PRT entries for PCI irq, but after commit 181380b7 ("PCI/ACPI:
Don't cache _PRT, and don't associate them with bus numbers"),
the list head for _PRT entries was removed, but left "list" in
struct acpi_prt_entry which is useless and stale, remove it now.

Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>