acpi_idle: delete bogus data from cpuidle_state.power_usage

The mW data in this field comes from AML _CST,
which was typed in by a BIOS writer, and is thus
considered unreliable.

Linux does not use it for making any decisions.
We do display it in sysfs where somebody might
read it and assume it is meaningful, so delete it.

Signed-off-by: Len Brown <len.brown@intel.com>