PCI / hotplug / ACPI: Drop unused acpiphp_debug declaration

Commit bd950799

 (PCI: acpiphp: Convert to dynamic debug) removed users
of acpiphp_debug variable and the variable itself but the declaration was
left in the header file. Drop this unused declaration.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>