Inline error tracker notification code

As we don't need the Discord functionality which brings quite a lot of
extra libraries/code

lib/music_library/application.ex

@@ -13,7 +13,7 @@ defmodule MusicLibrary.Application do
     _ = Req.RateLimiter.new()
 
     children = [
-      {ErrorTrackerNotifier, []},
+      MusicLibrary.ErrorNotifier,
       MusicLibrary.Vault,
       MusicLibrary.Repo,
       MusicLibrary.BackgroundRepo,