ACPI: Remove unused extern declaration acpi_paddr_to_node()

This is never used since commit 1e3590e2 ("[PATCH] pgdat allocation
for new node add (get node id by acpi)").

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