Remove unused type alias.

This is part of the base::Callback migration.

Context: https://cs.chromium.org/chromium/src/docs/callback.md?rcl=9fcc3764aea8f97e9f6de4a9ee61d554e67edcda&l=40
Change-Id: I7bc713213889d35e0868821a41c0ea9c8e0c16d1
Bug: 714018
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2030953Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Jose Lopes <jabolopes@google.com>
Auto-Submit: Jose Lopes <jabolopes@google.com>
Cr-Commit-Position: refs/heads/master@{#738233}

chrome/browser/download/download_history_unittest.cc

@@ -194,10 +194,6 @@ class TestDownloadHistoryObserver : public DownloadHistory::Observer {
 
 class DownloadHistoryTest : public testing::Test {
  public:
-  // Generic callback that receives a pointer to a StrictMockDownloadItem.
-  using DownloadItemCallback =
-      base::Callback<void(download::MockDownloadItem*)>;
-
   DownloadHistoryTest()
       : manager_(std::make_unique<content::MockDownloadManager>()) {}