Hand off comparison logic for database query to implementors

Equals overrides are not used in EF queries without running the comaprison locally instead of on the database, so to preserve that the comparison logic is instead implemented on a per manager basis.

osu.Game/Beatmaps/BeatmapManager.cs

@@ -145,6 +145,8 @@ namespace osu.Game.Beatmaps
             void resetIds() => beatmapSet.Beatmaps.ForEach(b => b.OnlineBeatmapID = null);
         }
 
+        protected override bool CheckLocalAvailability(BeatmapSetInfo model, IQueryable<BeatmapSetInfo> items) => items.Any(b => b.OnlineBeatmapSetID == model.OnlineBeatmapSetID);
+
         /// <summary>
         /// Delete a beatmap difficulty.
         /// </summary>