xen/setup/pm/acpi: Remove the call to boot_option_idle_override.

We needed that call in the past to force the kernel to use
default_idle (which called safe_halt, which called xen_safe_halt).

But set_pm_idle_to_default() does now that, so there is no need
to use this boot option operand.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>