ACPI / hotplug / PCI: Do not clear event callback pointer for docks

After recent changes adding dock station handling to the ACPI hotplug
core, it is not necessary to clear the .event() pointer in the
ACPIPHP device hotplug context for dock stations any more, so don't
do that.

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