Move MaxCombo to base DifficultyAttributes

osu.Game.Rulesets.Catch/Difficulty/CatchDifficultyAttributes.cs

@@ -8,6 +8,5 @@ namespace osu.Game.Rulesets.Catch.Difficulty
     public class CatchDifficultyAttributes : DifficultyAttributes
     {
         public double ApproachRate;
-        public int MaxCombo;
     }
 }

osu.Game.Rulesets.Osu/Difficulty/OsuDifficultyAttributes.cs

@@ -11,6 +11,5 @@ namespace osu.Game.Rulesets.Osu.Difficulty
         public double SpeedStrain;
         public double ApproachRate;
         public double OverallDifficulty;
-        public int MaxCombo;
     }
 }

osu.Game.Rulesets.Taiko/Difficulty/TaikoDifficultyAttributes.cs

@@ -8,6 +8,5 @@ namespace osu.Game.Rulesets.Taiko.Difficulty
     public class TaikoDifficultyAttributes : DifficultyAttributes
     {
         public double GreatHitWindow;
-        public int MaxCombo;
     }
 }

osu.Game/Rulesets/Difficulty/DifficultyAttributes.cs

@@ -12,6 +12,7 @@ namespace osu.Game.Rulesets.Difficulty
         public Skill[] Skills;
 
         public double StarRating;
+        public int MaxCombo;
 
         public DifficultyAttributes()
         {