Remove the url! plugin.

In rust-url 1.0 the `Url` struct is going to have private fields, and there
is no way to to create an aribitrary one without going through the parser.

The plugin never had a clear demonstrated performance benefit,
it was made mostly because it was possible and relatively easy at the time.

components/script/script_thread.rs

@@ -1880,7 +1880,7 @@ impl ScriptThread {
         };
 
         if load_data.url.scheme == "javascript" {
-            load_data.url = url!("about:blank");
+            load_data.url = Url::parse("about:blank").unwrap();
         }
 
         resource_thread.send(ControlMsg::Load(NetLoadData {