update: hide 'flush ghost peers' button when external tracker is enabled

The button doesn't do anything anyways for the external trackers. Users have to just wait it out.

app/Http/Controllers/Staff/FlushController.php

@@ -45,6 +45,10 @@ class FlushController extends Controller
      */
     public function peers(): \Illuminate\Http\RedirectResponse
     {
+        if (config('announce.external_tracker.is_enabled')) {
+            return redirect()->back()->withErrors("The external tracker doesn't support flushing peers.");
+        }
+
         $carbon = new Carbon();
         $peers = Peer::select(['torrent_id', 'user_id', 'peer_id', 'updated_at'])->where('updated_at', '<', $carbon->copy()->subHours(2))->get();