clean observer

packages/svelte/tests/runtime-runes/samples/class-directive-mutations/main.svelte

@@ -1,4 +1,6 @@
 <script>
+	import { onDestroy } from "svelte";
+
 	let {
         clazz = 'custom',
         foo = true,
@@ -32,6 +34,8 @@
         return result;
     }
 
+    onDestroy(() => { if (observer) observer.disconnect(); });
+
 </script>
 
 <main id="main" class:browser>