Return BPM back to OnlineInfo

Revert commit of "Move BPM out of OnlineInfo"

osu.Game/Beatmaps/BeatmapSetInfo.cs

@@ -35,11 +35,6 @@ namespace osu.Game.Beatmaps
         [NotMapped]
         public BeatmapSetMetrics Metrics { get; set; }
 
-        /// <summary>
-        /// The beats per minute of this beatmap set's song.
-        /// </summary>
-        public double BPM { get; set; }
-
         public double MaxStarDifficulty => Beatmaps?.Max(b => b.StarDifficulty) ?? 0;
 
         public double MaxLength => Beatmaps?.Max(b => b.Length) ?? 0;