[unity] Fix unneeded Unity 5 compiler condition.

Also fixes compatibility with Unity 2017

spine-unity/Assets/spine-unity/Mesh Generation/SpineMesh.cs

@@ -32,11 +32,9 @@ using UnityEngine;
 
 namespace Spine.Unity {
 	public static class SpineMesh {
-#if UNITY_5
+
 		internal const HideFlags MeshHideflags = HideFlags.DontSaveInBuild | HideFlags.DontSaveInEditor;
-#else
-		internal const HideFlags MeshHideflags = HideFlags.DontSave; 
-#endif
+
 		/// <summary>Factory method for creating a new mesh for use in Spine components. This can be called in field initializers.</summary>
 		public static Mesh NewMesh () {
 			var m = new Mesh();