sched: remove load_idx effect

Shortcut to remove rq->cpu_load[load_idx] effect in scheduler.
All other place rq->cpu_load used cpu_load[0].

Signed-off-by: Alex Shi <alex.shi@linaro.org>
sched: remove rq->cpu_load[load_idx]

Remove 5 kinds of load_idx from rq->cpu_load, busy_idx, idle_idx,
newidle_idx, wake_idx, forkexed_idx.

Signed-off-by: Alex Shi <alex.shi@linaro.org>
sched: clean up __update_cpu_load

Since we don't decay the rq->cpu_load, so we don't need the
pending_updates. But we still want update rq->rt_avg.