Fix potential null reference when loading background

As seen in
https://discordapp.com/channels/188630481301012481/188630652340404224/772094427342569493.
Caused due to async load of the loader, which means it may not be ready
before Next() is called.

osu.Game/Graphics/Backgrounds/SeasonalBackgroundLoader.cs

@@ -15,7 +15,6 @@ using osu.Game.Online.API.Requests.Responses;
 
 namespace osu.Game.Graphics.Backgrounds
 {
-    [LongRunningLoad]
     public class SeasonalBackgroundLoader : Component
     {
         /// <summary>