LibWeb: Don't cache property accesses on WindowProxy

Since the underlying HTML::Window can change, caching property accesses
on WindowProxy is not as simple as remembering the shape. Let's disable
caching here for now. We can come back to it in the future when we have
no low-hanging fruit left. :^)

Fixes an assertion failure on https://twinings.co.uk/

Tests/LibWeb/Text/input/window-proxy-property-inline-cache.html

@@ -0,0 +1,13 @@
+<script src="include.js"></script>
+<script>
+    test(() => {
+        function ic(o) {
+            return o.foo
+        }
+
+        window.foo = 123
+        println(ic(window))
+        delete window.foo
+        println(ic(window))
+    });
+</script>