cpuidle: menu: Cleanup after loadavg removal

The performance impact of loadavg was removed with commit a7fe5190
("cpuidle: menu: Remove get_loadavg() from the performance multiplier")
With only iowait remaining the description can be simplified, remove
also the no longer needed includes.

Signed-off-by: Christian Loehle <christian.loehle@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>