Refrain from attempting to clear skin sources in disposal

`Drawable.Dispose` is usually in an asynchronous context (async disposals stuff) and therefore this could cause a "collection was modified; enumeration opeartion may not execute" exception.

osu.Game/Skinning/SkinProvidingContainer.cs

@@ -210,7 +210,8 @@ protected override void Dispose(bool isDisposing)
             if (fallbackSource != null)
                 fallbackSource.SourceChanged -= OnSourceChanged;
 
-            SkinSources.Clear();
+            foreach (var source in SkinSources.OfType<ISkinSource>())
+                source.SourceChanged -= OnSourceChanged;
         }
 
         private class NoFallbackProxy : ISkinSource