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/util/opts.rs

@@ -463,7 +463,7 @@ const DEFAULT_USER_AGENT: UserAgent = UserAgent::Desktop;
 pub fn default_opts() -> Opts {
     Opts {
         is_running_problem_test: false,
-        url: Some(url!("about:blank")),
+        url: Some(Url::parse("about:blank").unwrap()),
         paint_threads: 1,
         gpu_painting: false,
         tile_size: 512,