Removed TypeHandle in Instance since it wasn't used anywhere. (#263)

Author: martindevans

src/Instance.cs

@@ -1,7 +1,6 @@
 using System;
 using System.Runtime.InteropServices;
 using System.Text;
-using Microsoft.Win32.SafeHandles;
 
 namespace Wasmtime
 {
@@ -603,21 +602,6 @@ internal Instance(Store store, ExternInstance instance)
             this.instance = instance;
         }
 
-        internal class TypeHandle : SafeHandleZeroOrMinusOneIsInvalid
-        {
-            public TypeHandle(IntPtr handle)
-                : base(true)
-            {
-                SetHandle(handle);
-            }
-
-            protected override bool ReleaseHandle()
-            {
-                Native.wasmtime_instancetype_delete(handle);
-                return true;
-            }
-        }
-
         private static class Native
         {
             [DllImport(Engine.LibraryName)]