Don't compute keymod difficulties for mania-specific beatmaps

These don't affect the number of keys in the beatmap at all for mania-specific beatmaps.

osu.Game.Rulesets.Mania/Difficulty/ManiaDifficultyCalculator.cs

@@ -29,9 +29,12 @@ namespace osu.Game.Rulesets.Mania.Difficulty
         /// </summary>
         private const double decay_weight = 0.9;
 
+        private readonly bool isForCurrentRuleset;
+
         public ManiaDifficultyCalculator(Ruleset ruleset, WorkingBeatmap beatmap)
             : base(ruleset, beatmap)
         {
+            isForCurrentRuleset = beatmap.BeatmapInfo.Ruleset.Equals(new ManiaRuleset().RulesetInfo);
         }
 
         protected override DifficultyAttributes Calculate(IBeatmap beatmap, Mod[] mods, double timeRate)
@@ -129,7 +132,22 @@ namespace osu.Game.Rulesets.Mania.Difficulty
             return difficulty;
         }
 
-        protected override Mod[] DifficultyAdjustmentMods => new Mod[]
+        protected override Mod[] DifficultyAdjustmentMods
+        {
+            get
+            {
+                if (isForCurrentRuleset)
+                {
+                    return new Mod[]
+                    {
+                        new ManiaModDoubleTime(),
+                        new ManiaModHalfTime(),
+                        new ManiaModEasy(),
+                        new ManiaModHardRock(),
+                    };
+                }
+
+                return new Mod[]
                 {
                     new ManiaModDoubleTime(),
                     new ManiaModHalfTime(),
@@ -145,6 +163,7 @@ namespace osu.Game.Rulesets.Mania.Difficulty
                     new ManiaModKey8(),
                     new ManiaModKey9(),
                 };
-
+            }
+        }
     }
 }