PCI/ACPI: Remove unused global list acpi_pci_roots

Now the global list acpi_pci_roots pci_root.c is useless, remove it.

Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Len Brown <lenb@kernel.org>