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/parser.rs

@@ -34,7 +34,7 @@ impl<'a> ParserContext<'a> {
 impl<'a> ParserContext<'a> {
     pub fn parse_url(&self, input: &str) -> Url {
         self.base_url.join(input)
-            .unwrap_or_else(|_| url!("about:invalid"))
+            .unwrap_or_else(|_| Url::parse("about:invalid").unwrap())
     }
 }