BatteryStats: Remove reading of cpu power

The cpu power reported by the uid_cputime kernel is inaccurate
and has only ever been recorded for dumping to batterystats.
The values have never been used in power blame calculations.

This change removes these power values which just cause noisy
data.

Bug: 36002715
Change-Id: I61bea9992aabb84d099689360fd9377b44b36e2f
Test: run `adb shell dumpsys batterystats`
Test: should not show `p=` for `Total cpu time:` line