Remove SystemBatteryConsumer

which has been superceded by AggegateBatteryConsumer

Bug: 186006876
Test: atest FrameworksCoreTests:com.android.internal.os.BatteryStatsTests
Test: Use BatteryStatsViewer to verify that it's still functional
Change-Id: I2e4fe36ff310b6d68923fcecdfd22ce865b1f32d