LibWeb: Don't override prototype on generated iterator prototypes

Generated iterator prototypes already have the IteratorPrototype as
their prototype, but we were incorrectly hijacking them and rerouting
to ObjectPrototype.

Regressed in cfe663435e.

Tests/LibWeb/Text/expected/url-search-params-iterator-iterator.txt

@@ -0,0 +1,6 @@
+[object Iterator]
+[object Iterator]
+true
+hello
+1
+[object Window]

Tests/LibWeb/Text/input/url-search-params-iterator-iterator.html

@@ -0,0 +1,13 @@
+<script src="include.js"></script>
+<script>
+    test(() => {
+        let u = new URLSearchParams();
+        let iterator = u[Symbol.iterator]();
+        println(iterator);
+        println(iterator[Symbol.iterator]());
+        println(iterator === iterator[Symbol.iterator]());
+        println(iterator[Symbol.iterator].call("hello"));
+        println(iterator[Symbol.iterator].call(1));
+        println(iterator[Symbol.iterator].call(window));
+    });
+</script>