Remove UNITY_2019_3_OR_NEWER directive

Assets/MackySoft/MackySoft.SerializeReferenceExtensions/Runtime/AddTypeMenuAttribute.cs

@@ -1,6 +1,4 @@
-#if UNITY_2019_3_OR_NEWER
-
-using System;
+using System;
 	
 /// <summary>
 /// An attribute that overrides the name of the type displayed in the SubclassSelector popup.
@@ -34,5 +32,4 @@ public sealed class AddTypeMenuAttribute : Attribute {
 		return (splittedDisplayName.Length != 0) ? splittedDisplayName[splittedDisplayName.Length - 1] : null;
 	}
 
-}
-#endif
+}

Assets/MackySoft/MackySoft.SerializeReferenceExtensions/Runtime/HideInTypeMenuAttribute.cs

@@ -1,5 +1,4 @@
-#if UNITY_2019_3_OR_NEWER
-using System;
+using System;
 
 /// <summary>
 /// An attribute that hides the type in the SubclassSelector.
@@ -7,5 +6,4 @@ using System;
 [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Enum | AttributeTargets.Interface, AllowMultiple = false, Inherited = false)]
 public sealed class HideInTypeMenuAttribute : Attribute {
 
-}
-#endif
+}

Assets/MackySoft/MackySoft.SerializeReferenceExtensions/Runtime/MackySoft.SerializeReferenceExtensions.asmdef

@@ -8,9 +8,7 @@
     "overrideReferences": false,
     "precompiledReferences": [],
     "autoReferenced": true,
-    "defineConstraints": [
-        "UNITY_2019_3_OR_NEWER"
-    ],
+    "defineConstraints": [],
     "versionDefines": [],
     "noEngineReferences": false
 }

Assets/MackySoft/MackySoft.SerializeReferenceExtensions/Runtime/SubclassSelectorAttribute.cs

@@ -1,5 +1,4 @@
-#if UNITY_2019_3_OR_NEWER
-using System;
+using System;
 using UnityEngine;
 
 /// <summary>
@@ -8,5 +7,4 @@ using UnityEngine;
 [AttributeUsage(AttributeTargets.Field,AllowMultiple = false)]
 public sealed class SubclassSelectorAttribute : PropertyAttribute {
 	
-}
-#endif
+}