Prerender: Remove deprecated prerender experiment histograms.

chrome/browser/predictors/autocomplete_action_predictor.cc

Index: chrome/browser/predictors/autocomplete_action_predictor.cc
diff --git a/chrome/browser/predictors/autocomplete_action_predictor.cc b/chrome/browser/predictors/autocomplete_action_predictor.cc
index a92ab887b82c856befe0a2008c9f72b6fa60a90f..34343c211ffeba8b2dbd6e101e5700640784af88 100644
--- a/chrome/browser/predictors/autocomplete_action_predictor.cc
+++ b/chrome/browser/predictors/autocomplete_action_predictor.cc
@@ -246,8 +246,7 @@ void AutocompleteActionPredictor::OnOmniboxOpenedUrl(const OmniboxLog& log) {
   }
 
   UMA_HISTOGRAM_BOOLEAN(
-      base::StringPrintf("Prerender.OmniboxNavigationsCouldPrerender%s",
-                         prerender::PrerenderManager::GetModeString()).c_str(),
+      "Prerender.OmniboxNavigationsCouldPrerender",
       prerender::IsOmniboxEnabled(profile_));
 
   const AutocompleteMatch& match = log.result.match_at(log.selected_index);