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/dom/htmliframeelement.rs

@@ -146,7 +146,7 @@ impl HTMLIFrameElement {
     pub fn process_the_iframe_attributes(&self) {
         let url = match self.get_url() {
             Some(url) => url.clone(),
-            None => url!("about:blank"),
+            None => Url::parse("about:blank").unwrap(),
         };
 
         self.navigate_or_reload_child_browsing_context(Some(url));