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/style/selector_matching.rs

@@ -69,7 +69,7 @@ lazy_static! {
             Ok(res) => {
                 Stylesheet::from_bytes(
                     &res,
-                    url!("chrome:///quirks-mode.css"),
+                    Url::parse("chrome:///quirks-mode.css").unwrap(),
                     None,
                     None,
                     Origin::UserAgent,