script: Do not run garbage collection when shutting down a Pipeline (#37691)

Removes a GC cleanup step on exit pipeline that took half a second at
the end
of the Speedmometer benchmark. As layout and script are now run serially
this
does not seem to be needed anymore. See also the very short discussion
on Zulip:
[#performance > GC in
handle_exit_pipeline](https://servo.zulipchat.com/#narrow/channel/416779-performance/topic/GC.20in.20handle_exit_pipeline/with/525697861).

Testing: Tested with Speedometer and the WPT tests should still work.

Signed-off-by: Narfinger <Narfinger@users.noreply.github.com>

components/script/dom/window.rs

@@ -2007,20 +2007,6 @@ impl Window {
             custom_elements.teardown();
         }
 
-        // The above code may not catch all DOM objects (e.g. DOM
-        // objects removed from the tree that haven't been collected
-        // yet). There should not be any such DOM nodes with layout
-        // data, but if there are, then when they are dropped, they
-        // will attempt to send a message to layout.
-        // This causes memory safety issues, because the DOM node uses
-        // the layout channel from its window, and the window has
-        // already been GC'd.  For nodes which do not have a live
-        // pointer, we can avoid this by GCing now:
-        self.Gc();
-        // but there may still be nodes being kept alive by user
-        // script.
-        // TODO: ensure that this doesn't happen!
-
         self.current_state.set(WindowState::Zombie);
         *self.js_runtime.borrow_mut() = None;