Remove str_to_string compile test

tests/compiletest/plugin/compile-fail/str_to_string.rs

@@ -1,16 +0,0 @@
-/* This Source Code Form is subject to the terms of the Mozilla Public
- * 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/. */
-
-#![feature(plugin)]
-#![plugin(plugins)]
-
-
-fn main() {
-    let x = "foo".to_string();
-    //~^ ERROR str.to_owned() is more efficient than str.to_string()
-
-    let x = &x[..];
-    x.to_string();
-    //~^ ERROR str.to_owned() is more efficient than str.to_string()
-}