add ability to unload class databases on their own

Author: nesrak1

AssetTools.NET/Extra/AssetsManager/AssetsManager.ClassDatabase.cs

@@ -1,7 +1,4 @@
-using System;
-using System.Collections.Generic;
-using System.IO;
-using System.Text;
+using System.IO;
 
 namespace AssetsTools.NET.Extra
 {
@@ -40,5 +37,15 @@ public ClassPackageFile LoadClassPackage(string path)
         {
             return LoadClassPackage(File.OpenRead(path));
         }
+
+        public void UnloadClassDatabase()
+        {
+            ClassDatabase = null;
+        }
+
+        public void UnloadClassPackage()
+        {
+            ClassPackage = null;
+        }
     }
 }