Remove unused callback using-declaration in BackgroundFetchDataManager

TBR=peter@chromium.org

Change-Id: Id0351debe9c0c99f885ce5fd324068279e050e4e
Reviewed-on: https://chromium-review.googlesource.com/1181601Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584556}

content/browser/background_fetch/background_fetch_data_manager.h

@@ -73,7 +73,6 @@ class CONTENT_EXPORT BackgroundFetchDataManager
                               const BackgroundFetchRegistration&)>;
   using NextRequestCallback =
       base::OnceCallback<void(scoped_refptr<BackgroundFetchRequestInfo>)>;
-  using NumRequestsCallback = base::OnceCallback<void(size_t)>;
 
   BackgroundFetchDataManager(
       BrowserContext* browser_context,