ACPI: Remove references to ACPI_STATE_S2 from acpi_pm_enter

Remove references to ACPI_STATE_S2, introduced by
acpi-implement-the-set_target-callback-from-pm_ops.patch, from acpi_pm_enter().

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Len Brown <len.brown@intel.com>