crash: Fix handlescope corruption on iframe load

In trying to fuzz test a different issue, I ran into a reproducible case where
we end up with a broken handlescope stack. The issue requires a large number
of handlescopes created with aggressive GC, so hopefully it isn't something
that too many users have run into.

The issue is that a single HandleScope address is used to initialize two
HandleScopes. The fix could just be to create a 2nd HS variable (to get a 2nd
address), but the first initialization is unnecessary and can just be removed.

(Note that EventManager dispatch creates its own HandleScope, so the one
removed in frameCompletedLoading really did nothing)

src/browser/Frame.zig

@@ -1006,11 +1006,8 @@ pub fn iframeCompletedLoading(self: *Frame, iframe: *IFrame) void {
         return;
     }
 
-    var ls: JS.Local.Scope = undefined;
-    self.js.localScope(&ls);
-    defer ls.deinit();
-
-    const entered = self.js.enter(&ls.handle_scope);
+    var hs: JS.HandleScope = undefined;
+    const entered = self.js.enter(&hs);
     defer entered.exit();
 
     blk: {