"using UnityEditor" is not necessary.

spine-unity/Assets/Plugins/Spine/AtlasAsset.cs

@@ -1,7 +1,6 @@
 using System;
 using System.IO;
 using UnityEngine;
-using UnityEditor;
 using Spine;
 
 public class AtlasAsset : ScriptableObject {

spine-unity/Assets/Plugins/Spine/SkeletonDataAsset.cs

@@ -2,7 +2,6 @@ using System;
 using System.IO;
 using System.Collections.Generic;
 using UnityEngine;
-using UnityEditor;
 using Spine;
 
 public class SkeletonDataAsset : ScriptableObject {