Remove unused constant history::kThumbnailsFilename.

components/history/core/browser/history_backend.cc

Index: components/history/core/browser/history_backend.cc
diff --git a/components/history/core/browser/history_backend.cc b/components/history/core/browser/history_backend.cc
index 4b767f84164c03b15202c80d925d2b73b180f523..c29fd9e7a9a4e77a727a38a2a0f6c228ab75de4f 100644
--- a/components/history/core/browser/history_backend.cc
+++ b/components/history/core/browser/history_backend.cc
@@ -280,10 +280,6 @@ void HistoryBackend::ClearCachedDataForContextID(ContextID context_id) {
   tracker_.ClearCachedDataForContextID(context_id);
 }
 
-base::FilePath HistoryBackend::GetThumbnailFileName() const {
-  return history_dir_.Append(kThumbnailsFilename);
-}
-
 base::FilePath HistoryBackend::GetFaviconsFileName() const {
   return history_dir_.Append(kFaviconsFilename);
 }
@@ -2572,9 +2568,6 @@ bool HistoryBackend::ClearAllThumbnailHistory(
     // fix the error if it exists. This may fail, in which case either the
     // file doesn't exist or there's no more we can do.
     sql::Connection::Delete(GetFaviconsFileName());
-
-    // Older version of the database.
-    sql::Connection::Delete(GetThumbnailFileName());
     return true;
   }