Stop reference-counting the ScriptTask.

The ScriptTask is owned by the proc in ScriptTaskFactory::create.

We started using a managed box for what would eventually become ScriptTask in
d21d27e08633b6a7f2b774a28c65c81d1eedbeaf.
Cleanup URL handling in ScriptTask::load a bit.

This should make it clearer that we always pass a URL to Document::new, and
avoids an unnecessary unwrap() call.
Remove the managed_boxes feature.
Remove Traceable from attr.rs
Remove Traceable from browsercontext.rs
Remove Traceable from characterdata.rs
Remove Traceable from customevent.rs
Remove Untraceable from dedicatedglobalworkerscope.rs
Remove Untraceable from canvasrenderingcontext2d.rs
Remove Untraceable/Traceable from document.rs
Remove Traceable from element.rs
Remove Traceable from event.rs
Remove Traceable from eventtarget.rs
Remove Traceable from formdata.rs
Remove Traceable from htmlcanvaselement.rs
Remove Traceable from htmliframelement.rs
Remove Untraceable from htmlimageelement.rs
Remove Traceable from messageevent.rs
Remove Traceable from mouseevent.rs
Remove Untraceable from treewalker.rs