Fix PP not being calculated correctly for nightcore scores

I guess this would be one way to roll forward with
ppy/osu#24640, but there is a chance something
somewhere else could have relied on the inheritance and might now break,
so not sure whether this should be the solution here...

Check is slightly rewritten since it wasn't making much sense to me
previously (why add nightcore to allowed mods if double time is
selected?)

PerformanceCalculator/LegacyHelper.cs

@@ -78,7 +78,7 @@ public static Mod[] ConvertToLegacyDifficultyAdjustmentMods(BeatmapInfo beatmapI
                                       .ToHashSet();
 
             // Special case to allow either DT or NC.
-            if (mods.Any(m => m is ModDoubleTime))
+            if (allowedMods.Any(type => type.IsSubclassOf(typeof(ModDoubleTime))) && mods.Any(m => m is ModNightcore))
                 allowedMods.Add(allMods.Single(m => m is ModNightcore).GetType());
 
             var result = new List<Mod>();

PerformanceCalculatorGUI/RulesetHelper.cs

@@ -49,7 +49,7 @@ public static Mod[] ConvertToLegacyDifficultyAdjustmentMods(Ruleset ruleset, Mod
                                       .ToHashSet();
 
             // Special case to allow either DT or NC.
-            if (mods.Any(m => m is ModDoubleTime))
+            if (allowedMods.Any(type => type.IsSubclassOf(typeof(ModDoubleTime))) && mods.Any(m => m is ModNightcore))
                 allowedMods.Add(allMods.Single(m => m is ModNightcore).GetType());
 
             var result = new List<Mod>();