Remove reduntant compile-fail test

Fixes https://github.com/rust-lang/rust/issues/19510

src/test/compile-fail/issue-17718-extern-const.rs

@@ -1,15 +0,0 @@
-// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
-// file at the top-level directory of this distribution and at
-// http://rust-lang.org/COPYRIGHT.
-//
-// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
-// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
-// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
-// option. This file may not be copied, modified, or distributed
-// except according to those terms.
-
-extern {
-    const FOO: uint; //~ ERROR: unexpected token: `const`
-}
-
-fn main() {}