Remove references to unused crates.

The cleanup is based on info from using "-W unused-extern-crates".

components/util/resource_files.rs

@@ -2,6 +2,7 @@
  * 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/. */
 
+#[cfg(not(target_os = "android"))]
 use std::env;
 use std::fs::File;
 use std::io::{self, Read};