update_engine: remove unused const.

BUG=chromium:309890
TEST=FEATURES="test" emerge-x86-generic update_engine passes.

Change-Id: Ib3c167522032abdc05232de8cf11d8c337552b51
Reviewed-on: https://chromium-review.googlesource.com/173828
Reviewed-by: David Zeuthen <zeuthen@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>
Commit-Queue: Yunlian Jiang <yunlian@chromium.org>
diff --git a/download_action.cc b/download_action.cc
index 89638c4..57070b5 100644
--- a/download_action.cc
+++ b/download_action.cc
@@ -25,9 +25,6 @@
 
 namespace chromeos_update_engine {
 
-// Use a buffer to reduce the number of IOPS on SSD devices.
-const size_t kFileWriterBufferSize = 128 * 1024;  // 128 KiB
-
 DownloadAction::DownloadAction(PrefsInterface* prefs,
                                SystemState* system_state,
                                HttpFetcher* http_fetcher)