Kill the plugins crate and its clippy support

Sometimes clippy gets outdated by months, and its current support setup
means that each Servo component need to opt into it by depending on
the plugins crate manually, and not all components do that.

components/constellation/lib.rs

@@ -2,13 +2,10 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+#![deny(unsafe_code)]
 #![feature(box_syntax)]
 #![feature(conservative_impl_trait)]
 #![feature(mpsc_select)]
-#![feature(plugin)]
-#![plugin(plugins)]
-
-#![deny(unsafe_code)]
 
 extern crate backtrace;
 extern crate bluetooth_traits;