statsd events/gauge: remove WallClockTime

EventMetricData stores wall_clock_timestamp_nanos.
It is expensive, costing 10 bytes per event and evidently not needed.
Similar for GaugeMetricData.

Bug: 113072343
Test: make -j8 statsd_test && adb sync data && adb shell data/nativetest64/statsd_test/statsd_test
Test: run cts-dev -m CtsStatsdHostTestCases
Test: Manually confirm that events/gauges don't have wallclock
Change-Id: Iae978a434354c049e1fa61d42536be981c862b4f