ACPI: Remove acpi_pci_slot_init() headers from internal.h

Since acpi_pci_slot_init() is now called from acpi_pci_init()
and pci-acpi.h contains its header, remove that header (and the
empty definition of that function for CONFIG_ACPI_PCI_SLOT unset)
from internal.h as it doesn't have to be there any more.  That also
avoids a build warning about duplicate function definitions for
CONFIG_ACPI_PCI_SLOT unset.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>