Use TransformImmediately instead at beatmap listing search control

Applies two changes:
 - Use `TransformImmediately` which achieves the same wanted transition behaviour without any issues.
 - Move the transition behaviour override into `BeatmapListingSearchControl` in a nested subclass of `UpdateableBeatmapSetCover`.

osu.Game/Beatmaps/Drawables/UpdateableBeatmapSetCover.cs

@@ -41,10 +41,6 @@ namespace osu.Game.Beatmaps.Drawables
         protected override DelayedLoadWrapper CreateDelayedLoadWrapper(Func<Drawable> createContentFunc, double timeBeforeLoad)
             => new DelayedLoadUnloadWrapper(createContentFunc, timeBeforeLoad);
 
-        // by default, ModelBackedDrawable hides the old drawable only after the new one has been fully loaded.
-        // this can lead to weird appearance if the cover is not fully opaque, so fade out as soon as a new load is requested in this particular case.
-        protected override void OnLoadStarted() => ApplyHideTransforms(DisplayedDrawable);
-
         protected override Drawable CreateDrawable(BeatmapSetInfo model)
         {
             if (model == null)

osu.Game/Overlays/BeatmapListing/BeatmapListingSearchControl.cs

@@ -90,7 +90,7 @@ namespace osu.Game.Overlays.BeatmapListing
                 {
                     RelativeSizeAxes = Axes.Both,
                     Masking = true,
-                    Child = beatmapCover = new UpdateableBeatmapSetCover
+                    Child = beatmapCover = new TopSearchBeatmapSetCover
                     {
                         RelativeSizeAxes = Axes.Both,
                         Alpha = 0,
@@ -184,5 +184,10 @@ namespace osu.Game.Overlays.BeatmapListing
                 return true;
             }
         }
+
+        private class TopSearchBeatmapSetCover : UpdateableBeatmapSetCover
+        {
+            protected override bool TransformImmediately => true;
+        }
     }
 }