Remove clock resolution page load histograms.

These were temporary metrics intended to understand whether high/low
resolution clocks adversely impact page load metrics. After collecting a few
months of data it was determined that clock resolution doesn't adversely
impact our metrics, and it that these histograms were no longer needed.

BUG=394757

Review-Url: https://codereview.chromium.org/2155143003
Cr-Commit-Position: refs/heads/master@{#406143}
void CorePageLoadMetricsObserver::OnLoadEventStart(
    const page_load_metrics::PageLoadTiming& timing,
    const page_load_metrics::PageLoadExtraInfo& info) {
  if (WasStartedInForegroundOptionalEventInForeground(timing.load_event_start,
                                                      info)) {
    PAGE_LOAD_HISTOGRAM(internal::kHistogramLoadImmediate,
                        timing.load_event_start.value());
  } else {
    PAGE_LOAD_HISTOGRAM(internal::kBackgroundHistogramLoadImmediate,
                        timing.load_event_start.value());
  }
}

void CorePageLoadMetricsObserver::OnLoadEventStart(
    const page_load_metrics::PageLoadTiming& timing,
    const page_load_metrics::PageLoadExtraInfo& info) {
  if (WasStartedInForegroundOptionalEventInForeground(timing.load_event_start,
                                                      info)) {
    PAGE_LOAD_HISTOGRAM(internal::kHistogramLoadImmediate,
                        timing.load_event_start.value());
  } else {
    PAGE_LOAD_HISTOGRAM(internal::kBackgroundHistogramLoadImmediate,
                        timing.load_event_start.value());
  }
}
