Remove duplicated histogram suffix declarations

https://crrev.com/c/826706 moved these constants to
histogram_suffixes.h in, but forgot to remove them from
data_reduction_proxy_metrics_observer.h.

Change-Id: I52bce11ac6ba1186a3a08fc71cda3bf22b431a34
Reviewed-on: https://chromium-review.googlesource.com/831376Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524651}

chrome/browser/page_load_metrics/observers/data_reduction_proxy_metrics_observer.h

@@ -28,17 +28,6 @@ namespace internal {
 // Various UMA histogram names for DataReductionProxy core page load metrics.
 extern const char kHistogramDataReductionProxyPrefix[];
 extern const char kHistogramDataReductionProxyLoFiOnPrefix[];
-extern const char kHistogramDOMContentLoadedEventFiredSuffix[];
-extern const char kHistogramFirstLayoutSuffix[];
-extern const char kHistogramLoadEventFiredSuffix[];
-extern const char kHistogramFirstContentfulPaintSuffix[];
-extern const char kHistogramFirstMeaningfulPaintSuffix[];
-extern const char kHistogramFirstImagePaintSuffix[];
-extern const char kHistogramFirstPaintSuffix[];
-extern const char kHistogramFirstTextPaintSuffix[];
-extern const char kHistogramParseStartSuffix[];
-extern const char kHistogramParseBlockedOnScriptLoadSuffix[];
-extern const char kHistogramParseDurationSuffix[];
 
 // Byte and request specific histogram suffixes.
 extern const char kResourcesPercentProxied[];