Remove IJsonSerializable interface

Was pretty pointless and made it hard to use the custom serialisation
terms arbitrarily in tests.

osu.Game/IO/Serialization/JsonSerializableExtensions.cs

@@ -0,0 +1,28 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System.Collections.Generic;
+using Newtonsoft.Json;
+using osu.Framework.IO.Serialization;
+
+namespace osu.Game.IO.Serialization
+{
+    public static class JsonSerializableExtensions
+    {
+        public static string Serialize(this object obj) => JsonConvert.SerializeObject(obj, CreateGlobalSettings());
+
+        public static T Deserialize<T>(this string objString) => JsonConvert.DeserializeObject<T>(objString, CreateGlobalSettings());
+
+        public static void DeserializeInto<T>(this string objString, T target) => JsonConvert.PopulateObject(objString, target, CreateGlobalSettings());
+
+        public static JsonSerializerSettings CreateGlobalSettings() => new JsonSerializerSettings
+        {
+            ReferenceLoopHandling = ReferenceLoopHandling.Ignore,
+            Formatting = Formatting.Indented,
+            ObjectCreationHandling = ObjectCreationHandling.Replace,
+            DefaultValueHandling = DefaultValueHandling.IgnoreAndPopulate,
+            Converters = new List<JsonConverter> { new Vector2Converter() },
+            ContractResolver = new KeyContractResolver()
+        };
+    }
+}