Do not serialise `SkinLayoutInfo.AllDrawables`

- It is entirely derived from `SkinLayoutInfo.DrawableInfo`, which is
  the actual primary thing we want to serialise.
- It will never get read out from any serialised files anyway (corollary
  of the previous point - it is a get-only property derived from another).
- It is only used in tests.

All of the three reasons above make serialising the property out to skin
files nothing more than a waste of space.

osu.Game/Skinning/SkinLayoutInfo.cs

@@ -19,6 +19,7 @@ public class SkinLayoutInfo
     {
         private const string global_identifier = @"global";
 
+        [JsonIgnore]
         public IEnumerable<SerialisedDrawableInfo> AllDrawables => DrawableInfo.Values.SelectMany(v => v);
 
         [JsonProperty]