Fix HitWindows getting serialized alongside HitObjects

These were being serialized as the base type. On deserialization, due to
the HitWindow of objects being non-null, they would not get correctly
initialised by the CreateHitWindows() virtual method.

- Closes #10403

osu.Game/Rulesets/Objects/HitObject.cs

@@ -77,6 +77,7 @@ namespace osu.Game.Rulesets.Objects
         /// <summary>
         /// The hit windows for this <see cref="HitObject"/>.
         /// </summary>
+        [JsonIgnore]
         public HitWindows HitWindows { get; set; }
 
         private readonly List<HitObject> nestedHitObjects = new List<HitObject>();